There is no thrill like the thrill you get from operating equipment you have built yourself. If you have never built a project from a magazine before, let this be your first--you'll see how much satisfaction and fun you can have!
The FM transmitter is designed run from a 9-volt battery and is made from readily available parts. The author's primary use is as a baby monitor, but uses of a transmitter like this one are almost limitless. It is very sensitive, and easily capable of picking up a conversation in any part of the room. The dimensions and values give here will allow static-free reception within the perimeter of most homes.
No license is required for this transmitter according to FCC regulations regarding wireless microphones. (The emissions must stay within a band of 200 KHz, its output between 88 and 108 MHz, and the field strength of the radiated emissions must not exceed 50uV/m at a distance of 15 meters (45 feet) from the device.)
If powered from a 9-volt battery and used with an antenna no longer than 12 inches, the transmitter's radiated power will be within the FCC limits. The FCC takes a dim view of persons operating outside the legal power limits, so please do not substitute any components in this circuit which would alter the output power.

Parts List:
All resistors are 1/8-watt, 5%.
R1,R6 = 1K C1,C3 = 10uF/25V, electrolytic
R2 = 15K C2 = 2.2uF/25V, electrolytic
R3 = 6K8 C4,C7 = 0.1uF/25V, ceramic
R4 = 10K C5 = 5 to 60pF, trimmer
R5,R7 = 4K7 C6 = hand-made (see text)
R8 = 2K2 Q1,Q2 = 2N2222(A), NPN transistor
R9 = 220 ohms L1 = hand-made (see text)
Miscellaneous: perforated construction board, 9-Volt battery, battery clip,
electret microphone, 24-gauge insulated wire, bare wire, solder.
Circuitry:
Take a look at the schematic above. Audio is picked up from the room by an electret microphone and amplified by Q1. Resistors R2-R5 set up the DC operating bias of Q1. Capacitor C3 serves to improve the AC response to the audio voltage, and C2 blocks the DC bias and couples the AC to the next stage, where the RF action takes place. The amplified AC voltage from Q1 is routed to the base of Q2. Transistor Q2 and associated circuitry (C5 and the inductor) form an oscillator that operates in the 80-130 MHz range, which is your regular dial on any FM radio.
The oscillator is voltage-controlled, so it is modulated by the audio voltage that is applied to the base of Q2. Resistor R6 limits the input to the RF section, and its value can be adjusted as necessary to limit the volume of the input. That will help control the amount of distortion you have on very loud inputs.
Resistors R7-R9 set the DC operating bias of Q2, another 2N2222 that's used as the oscillator and modulator of the transmitter. Capacitor C5 is a 6-50pF trimmer capacitor that's used to tune the oscillator tank circuit, and C4 routes the RF from the oscillator to ground to prevent unstable operation.
Completed Prototype Construction:
The FM transmitter is built on a piece of perforated construction board (a.k.a. 'perf-board') with 0.1-inch hole spacing. Component spacing is not critical, but placement is. You should place the components on the board in a layout that is similar to the prototype shown. Generally, you will also want to make the transmitter as small as possible.
Let's start from the left side of the schematic and work o the tright. You'll want to cut out a piece of perfboard that is 12 holes wide and 30 holes long. That will give you plenty of room to work with, but still produce a small unit. First lay out two power lines on the board with bare wire; the positive supply from the battery will be on top, and the negative (ground) will be on the bottom.
A 1K resistor (R1) supplies the bias voltage for the microphone. Remember to install the resistor vertically, next to the positive supply line, and bend the other end of the lead to the board. Go through the board and down toward the ground bus. Now insert the micophone leads into the board, making sure that the ground lead of the microphone can be soldered to the ground bus on the board. Route the lead from R1 to the positive lead of the microphone and solder it. The 10-uF capacitor, C1, should be placed in the middle of the board, oriented as shown on the schematic, and soldered to the microphone/R1 junction.
This project requires two hand-made parts--coil L1 and capacitor C6--but you make both of them yourself using only wire and a common pencil for the coil form. The inductor ismade by winding two pieces of 24-gauge insulated wire, laid side-by-side, around the pencil six times. Remove the coil you have formed and unscrew the two coils apart from each other. One of these coils, the better-looking of the two, will be used in the tank circuit (L1) and the other can be used in the next one you build.
The other hand-made component, capacitor C6, is part of the oscillator feedback. To make this small value capacitor, take a 4-inch piece of 24-gauge insulated wire, bend it over double and, beginning 1/2-inch from the open end, twist the wire as if you were forming a rope. When you have about 1 inch of twisted wire, stop and cut the looped end off leaving about 1/2-inch of twisted wire (this forms the capacitor) and 1/2-inch of untwisted wire for leads.
Capacitor C7, a 0.1uF capacitor, is one of the most critical components in the circuit. You must place it across the L1-Q2-R9 assembly, as shown in Fig. 1, to reduce the amout of RF feedback you'll get iinto the rest of the circuit. The antenna (more 24-gauge wire) shoucl be soldered to the coil you made, about 2-turns up from the bottom, or the transistor side, and should be about 8-12-inches long.
Operation:
To use the transmitter, set up a radio in the area at least 10 feet (3 meters) from the project. Find a blank spot on the FM dial and turn the radio up so you can hear the static.
Connect a 9-volt battery to the transmitter and listed to the radio. Slowly adjust the tank capacitor (C5) until you "quiet" the receiver; this is the tuned spot. Note that when you move your hand from the transmitter, you will detune the circuit somewhat. It is usually best to leave it detuned and tune the radio in to get the best reception. If you et the tuning randge you desire, you can squeeze the coils in the tank circuit closer together to raise the frequency, or pull them apart just a little to lower it.
The circuit works best when powered by a battery, but if a wall adapter is required, make certain that the ripple voltage is as low as possible, or you will get hum in the receiver. R-E

FM Transmitter (up to 500 meter range)

Parts List:
Resistors are 1/4 Watt, 5%
R1 = 10K
R2 = 86K
R3 = 3K3 (3.3K)
R4 = 2K2
R5 = 100 ohm
Capacitors:
C1,C8 = 2N2 (2.2nF), ceramic
C2 = 4N7, ceramic
C3,C6 = 10pF miniature variable
C4,C9 = 100pF, ceramic
C5 = 1uF, 25V, electrolytic
C7 = 1uF, polyester
Semiconductors:
Q1 = BC108, NTE123A, NPN transistor
Q2 = 2N2369, 2N2222, NTE123A, NPN transistor
Notes:
Q1 and Q2 both reference to a NTE123A replacement type.
With a short antenna the maximum distance is about 600 feet (200M). But with a proper antenna this distance can easily
be changed to about 1/2 mile (800M)! Even so, keep in mind that this simple transmitter is just that; simple!
If you find that the frequency drift a little, secure the coil with candle wax or a dab of hot-melt glue. The coil, L1,
is wound on a 10mm body. Use a wooden dowel or similar. Wind about 4 turn of enameled copper wire
(also called 'magnet wire'). Obviously, remove the body after winding. Scrape the enamel off both ends of the coil
before attempting to solder.
As for the ceramic capacitors, try to find and use the socalled 'NP0' types for low noise/temperature drift. They
have a black stripe at the top of the cap. They are manufactured by Philips.
The variable capacitors, C3 and C6, are just the regular trimmer caps.
This type of transmitter works best in a metal enclosure. Just make sure nothing of your circuit is touching it.
"Gnd." in the schematic diagram just refers to the negative pole of your battery.

4 Transisitor Transmiter

Parts List:
All resistors are 1/4 watt 5%
R1,R2,R8 = 1K C1 = 1uF/63V, electrolytic
R3 = 100K C2,C3 = 10nF, ceramic
R4 = 150K C4,C5,C9 = 4.7uF/63V, electrolytic
R5,R7 = 10K C6,C12,C13,C14 = 1nF, ceramic
R6 = 220 ohm C7,C8,C11 = 5pF, ceramic
R9 = 10 ohm C10 = 220uF/63V, electrolytic
P1 = 5K trimpot
Q1,Q2 = 2N3904 L1 = 3.9uH
Q3,Q4 = 7001, NTE123AP L2 = 1uH
D1 = 1N4002 L3 = aircoil, 8.5 turns air space
1/4 inch diameter
Circuit Notes:
This circuit provides an FM modulated signal with an output power of around 500mW. The input microphone pre-amp is built around a couple of 2N3904 transistors (Q1/Q2), and audio gain is limited by the 5k preset trim potentiometer.
The oscillator is a Colpitts stage, frequency of oscillation governed by the tank circuit made from two 5pF ceramic capacitors and the L2 inductor.
(Click here for Colpitts Oscillator Resonant Frequency Equation.)
Frequency is around 100Mhz with values shown.
Audio modulation is fed into the tank circuit via the 5p capacitor, the 10k resistor and 1N4002 controlling the amount of modulation. The oscillator output is fed into the 3.9uH inductor (L1) which will have a high impedance at RF frequencies.
The output stage operates as a 'Class D' amplifier, no direct bias is applied but the RF signal developed across the 3.9uH inductor is sufficient to drive this stage. The emitter resistor and 1k base resistor prevent instability and thermal runaway in this stage.

Miniature FM Transmiter 2N2222

Parts List:
R1,R3 = 100K
R2 = 10K
R4 = 470 ohm
C1,C4 = 470pF
C2,C3 = 4.7µF, 16V, electrolytic
C5,C6 = 4.7pF
C7 = 4-40pF trimmer cap (optional, see text)
L1 = 1µH
Q1,Q2 = 2N2222, NPN transistor
Mic = Electret Microphone
B1 = 9 Volt, Alkaline battery
Notes:
Nothing critical here. To get a bit of tuning out of the coil you could put a 4-40pF trimmer capacitor (optional) parallel over the 1 µH coil, L1.
C1/C4 and C5/C6 are ceramic capacitors, preferably NPO (low noise) types. C2/C3 are electrolytic or can be tantalum types.
If you decide to substitute transistors with something similar you already have, it maybe necessary adjust the collector voltage of Q1 by changing the value of R2 or R3 (because you change transistors, it changes this bias on the base of Q1). It should be about 1/2 the supply voltage (about 4 or 5v).
The antenna is nothing more than a piece of 12" wire or a piece of piano wire from 6" to 12".
To find the signal on your standard FM Radio dial, make sure there is a signal coming into the microphone, otherwise the circuit won't work. I use an old mechanical alarm clock (you know, with those two large bells on it). I put this clock by the microphone which picks up the loud tick-tock. I'm sure you get the idea... Or you can just lightly tap the microphone while searching for the location of the signal on your receiver. Most of all...BE PATIENT!

Sensitive FM Transmiter

Parts List:
R1,R3 = 10K C1,C2 = 0.1uF Q1,Q2 = 2N3904
R2 = 100K C3 = 0.001uF L1 = see text
R4 = 4.7K C4 = 10-40pF
R5 = 330 ohm C5 = 4.7pF
Construction:
L1 is 0.1uH, 6 to 8 turns of 22-gauge hookup wire close wound around a 1/4-inch diameter non-conductive core such as a pencil.
The default for the capacitors type is ceramic, preferably the npo 1% type or equivalent. But basically nothing critical here. Use any capacitor you have laying around, but no electrolytic or tantalum caps. Only if you intend to use this circuit outside the home you may want to select more temperature stable capacitors like polyester types. Again, don't go out of your way to buy stuff, use what ever you can salvage.
If you decide to substitute transistors with something similar you already have, it maybe necessary adjust the collector voltage of Q1 by changing the value of R2 or R3 (because you change transistors, it changes this bias on the base of Q1). It should be about 1/2 the supply voltage (about 4 or 5v).
To find the signal on your receiver, make sure there is a signal coming into the microphone, otherwise the circuit won't work. I use an old mechanical alarm clock (you know, with those two large bells on it). I put this clock by the microphone which picks up the loud tick-tock. I'm sure you get the idea... Or you can just lightly tap the microphone while searching for the location of the signal on your receiver.

Miniature FM ( Voice ) Transmiter

Parts List:
R1 = 4.7K Q1 = 2N3904
R2 = 330 ohm L1 = see text
C1 = 0.001uF (1nF) Electret mike, antenna, 3V battery (button cell)
C2 = 10-40pF
C3 = 4.7pF
Construction:
This is another easy-to-build miniature transmitter that uses a minimum of parts. Construction is straight forward and non-critical. Although this design uses a 3-volt power source (such as a lithium coin or button cell), a 9-volt battery can be used, instead, by increasing the value of R1 to 15K and R2 to 1K. C4 is an optional RF bypass capacitor that may help improve performance and increase the range a bit. Experiment to find best results.
L1 was made by stripping 22 gauge hookup wire of it's insulation, then wrapping it in the grooves of the screw threads of a 1/4 diameter bolt, and then back-screwing the bolt out of the resulting coil. 8 turns were made around the bolt. By wrapping the turns in the threads, a uniform seperation was made between the coil windings.
If you decide to substitute transistors with something similar you already have, it maybe necessary adjust the collector voltage of Q1 by changing the value of R2 or R3 (because you change transistors, it changes this bias on the base of Q1). It should be about 1/2 the supply voltage (about 4 or 5v).
Notes:
The default for the capacitors type is ceramic, preferably the npo 1% type or equivalent. But basically nothing critical here. Use any capacitor you have laying arround, but no electrolytic or tantalum caps. Don't go out and rush to the store. Most parts can be salvaged from somewhere. Only if you intend to use this circuit outside the home you may want to select more temperature stable capacitors.
I'm not sure about the range. With the 3V supply it is probably around 100 feet or so. The 9V supply will beef up the range considerably, again not tested, but probably in the 300 feet range or so.
To find the signal on your receiver, make sure there is a signal coming into the microphone, otherwise the circuit won't work. I use an old mechanical alarm clock (you know, with those two large bells on it). I put this clock by the microphone which picks up the loud tick-tock. I'm sure you get the idea... Or you can just lightly tap the microphone while searching for the location of the signal on your receiver.

Miniature FM Transmiter

Parts List
R1,R4,R6 = 10K C1,C2 = 0.1uF Q1,Q2 = 2N3904
R2 = 1M C3 = 0.01uF L1 = 0.1uH
R3 = 100K C4 = 4-40pF
R5 = 100 ohm C5 = 4.7pF
R7 = 1K
Construction
This miniature transmitter is easy to construct and it's transmissions can be picked up on any standard FM radio. It has a range of up to 1/4-mile (400 meters) or more, depending on the line-of-sight, obstructions by large buildings, etc. It is great for room monitoring, baby listening, nature research, etc.
L1 is 8 to 10 turns of 22 gauge hookup wire close wound around a non-conductive 1/4-inch diamter form, such as a pencil.
C4 is a small, screw-adjustable, trimmer capacitor.
Set your FM radio for a clear, black space in the lower end of the band (88MHz). Then, with a non-metallic/non-conductive trimmer tool, adjust this capacitor for the clearest reception. A little experimenting and patience may be in order.
Most of the parts values are not critical, so you can try adjusting them to see what happens.
If you decide to substitute transistors with something similar you already have, it maybe necessary adjust the collector voltage of Q1 by changing the value of R2 or R3 (because you change transistors, it changes this bias on the base of Q1). It should be about 1/2 the supply voltage (about 4 or 5v).
Notes
The default for the capacitors type is ceramic, preferably the npo 1% (low noise) type or equivalent. But basically nothing critical here. Use any capacitor you have laying around, but NO electrolytic or tantalum caps. Only if you intend to use this circuit outside the home you may want to select more temperature stable capacitors.
To find the signal on your receiver, make sure there is a signal coming into the microphone, otherwise the circuit won't work. I use an old mechanical alarm clock (you know, with those two large bells on it). I put this clock by the microphone which picks up the loud tick-tock. I'm sure you get the idea... Or you can just lightly tap the microphone while searching for the location of the signal on your receiver.

The circuit transmits on Medium Wave (this is the small problem with the police). IC1a, together

with a sensor (try a 20cm x 20cm sheet of tin foil) oscillates at just over 1MHz. This is modulated by an audio frequency (a continuous beep) produced by IC1b. When a hand or a foot approaches the sensor, the frequency of the transmitter (IC1a) drops appreciably. Suppose now that the circuit transmits at 1MHz. Suppose also that your radio is tuned to a frequency just below this. The 1MHz transmission will therefore not be heard by the radio. But bring a hand or a foot near to the sensor, and the transmitter's frequency will drop, and a beep will be heard from the radio. Attach the antenna to a multiplug adapter that is plugged into the mains, and you will find that the Medium Wave transmission radiates from every wire in your house. Now place a suitably tuned Medium Wave radio near some wires or a plug point in your house, and an early-warning system is set up. Instead of using the sheet of tin foil as the sensor, you could use a doorknob, or burglar bars. Or you could use a pushbutton and series resistor (wired in series with the 33K resistor - the pushbutton would short it out) to decrease the frequency of IC1a, so activating the system by means of a pushbutton switch. In this case, the radio would be tuned to a frequency just below that of the transmitter.

BUZZER

This novel buzzer circuit uses a relay in series with a small audio transformer and speaker. When the switch is pressed, the relay will operate via the transformer primary and closed relay contact. As soon as the relay operates the normally closed contact will open, removing power from the relay, the contacts close and the sequence repeats, all very quickly...so fast that the pulse of current causes fluctuations in the transformer primary, and hence secondary. The speakers tone is thus proportional to relay operating frequency. The capacitor C can be used to "tune" the note. The nominal value is 0.001uF, increasing capacitance lowers the buzzers tone.

MOTOR ALARM

Notes:Any number of normally open switches may be used. Fit the mercury switches so that they close when the steering is moved or when the bike is lifted off its side-stand or pushed forward off its centre-stand. Use micro-switches to protect removable panels and the lids of panniers etc. While at least one switch remains closed, the siren will sound. About two minutes after the switches have been opened again, the alarm will reset. How long it takes to switch off depends on the characteristics of the actual components used. But, up to a point, you can adjust the time to suit your requirements by changing the value of C1.The circuit board and switches must be protected from the elements. Dampness or condensation will cause malfunction. Without its terminal blocks, the board is small. Ideally, you should try to find a siren with enough spare space inside to accommodate it. Fit a 1-amp in-line fuse close to the power source. This protects the wiring. Instead of using a key-switch you can use a hidden switch; or you could use the normally closed contacts of a small relay. Wire the relay coil so that it is energized while the ignition is on. Then every time you turn the ignition off, the alarm will set itself.When it's not sounding, the circuit uses virtually no current. This should make it useful in other circumstances. For example, powered by dry batteries and with the relay and siren voltages to suit, it could be fitted inside a computer or anything else that's in danger of being picked up and carried away. The low standby current and automatic reset means that for this sort of application an external on/off switch may not be necessary.The Support Material for this alarm includes a detailed guide to the construction of the circuit-board, a parts list, a complete circuit description and more.

WATER LEVEL ALARM

The conductance of fluids:
Conductance is the reciprocal of resistance. The conductance of fluids vary with temperature, volume and separation distance of the measurement probes. Tap water has a conductance of about 50 uS / cm measured at 25 C. This is 20k/cm at 25 C. See this site for more details about the conductance of fluids.

Notes:
This circuit will trigger with any fluid with a resistance under 900K between the maximum separation distance of the probes. Let me explain further. The circuit uses a 4050B CMOS hex buffer working on a 5 volt supply. All gates are biased off by the 10M resistors connected between ground and buffer input. The "common" probe the topmost probe above probe 1 in the diagram above is connected to the positive 5 volt supply. If probe 1 is spaced 1 cm away from the common probe and tap water at 25 C is detected between the probes (a resistance of 20k) then the top gate is activated and the LED 1 will light. Similarly if probe 2 at 2 cm distance from the common probe detects water, LED 2 will light and so on. Switch 1 is used to select which output from the hex buffer will trigger the audible oscillator made from the gates of a CMOS 4011B IC.

Placement of Probes:
As 7 wires are needed for the probe I reccommend the use of 8 way computer ribbon cable. The first two wires may be doubled and act as the common probe wire. Each subsequent wire may be cut to required length, if required a couple of millimetres of insulation may be stripped back, though the open "cut off" wire end should be sufficient to act as the probe. The fluid and distance between probe 6 and the common probe wire must be less than 900k. This is because any voltage below 0.5 Volt is detected by the CMOS IC as logic 0. A quick potential check using a 900k resistance and the divider formed with the 10M resistor at the input proves this point:
5 x (0.9 / (0.9+10) = 0.41 Volt.
As this voltage is below 0.5 volt it is interpreted as a logic 0 and the LED will light. If measuring tap water at 25 C then the distance between top probe and common may be up to 45 cm apart. For other temperatures and fluids, it is advisable to use an ohmmeter first. When placing the probes the common probe must be the lowest placed probe, as the water level rises, it will first pass probe 1, then 2 and finally probe 6.

ALARM

Circuit Notes:
Each zone uses a normally closed contact. These can be micro switches or standard alarm contacts (usually reed switches). Suitable switches can be bought from alarm shops and concealed in door frames, or window ledges.

Zone 1 is a timed zone which must be used as the entry and exit point of the building. Zones 2 - 5 are immediate zones, which will trigger the alarm with no delay. Some RF immunity is provided for long wiring runs by the input capacitors, C1-C5. C7 and R14 also form a transient suppresser. The key switch acts as the Set/Unset and Reset switch. For good security this should be the metal type with a key.

Operation:
At switch on, C6 will charge via R11, this acts as the exit delay and is set to around 30 seconds. This can be altered by varying either C6 or R11. Once the timing period has elapsed, LED6 will light, meaning the system is armed. LED6 may be mounted externally (at the bell box for example) and provides visual indication that the system has set. Once set any contact that opens will trigger the alarm, including Zone 1. To prevent triggering the alarm on entry to the building, the concealed re-entry switch must be operated. This will discharge C6 and start the entry timer. The re-entry switch could be a concealed reed switch, located anywhere in a door frame, but invisible to the eye. The panic switch, when pressed, will trigger the alarm when set. Relay contacts RLA1 provide the latch, RLA2 operate the siren or buzzer