AM Radio Transmitter Using 555 Chip
This project shows how to build a simple AM radio transmitter based on 555 timer IC. The circuit parts are: the 555 timer IC, a NPN transistor three caps, three resistors and a potentiometer. The circuit is able to generate an amplitude modulation signal at 600Khz and you are able to receive it using a plain AM receiver. The range is about 30-40 feet.
You are going to build an AM radio transmitter AND you will be shown how it works. When you finish your radio, it will look something like mine in the picture above.
These are the components you will need:
– 555 timer chip
– NPN transistor
– two #103 capacitors (0.01 microfarads or 10,000 picofarads)
– #102 capacitor (0.001 microfarads or 1,000 picofarads)
– some short wires
– two 1 Kilohm resistors
– 10 Kilohm resistor
– 1/8 inch (3.5 millimeter) female audio jack (yours may have more or less than three wires, but it must have at least two)
– 5 Kilohm potentiometer
– 1/8 inch (3.5 millimeter) male audio cable
– AM radio receiver
– Antenna. Yours doesn’t have to be made out of a pop can, but the pop can works
Test your radio!
To test the AM radio transmitter, simply set the antenna next to your AM radio receiver (Alarm clock) tuned to approximately 600 KHz. Then play with the potentiometer until you can hear your music on your radio. The frequency generated by this devise will be anywhere from 100-480 Kilohertz if you used all the correct component values.
If you hear weird sounds when you turn the potentiometer (and do not hear the audio signal) That means that your radio is working, but your audio signal needs to be configured. try turning the volumed of your audio signal up.
~Is there power applied to your transmitter?
~Is the audio signal on?
~try turning the potentiometer.
~try turning up the audio signal
So how does this work?
The audio signal is controlling when the radio signal is being transmitted using its amplitude. this is called amplitude modulation (hence A.M. Radio) (See picture above). However, as kr.baker pointed out in the comments, “The audio is “modulating” the RESET pin on the 7555. This means that the signal is turning the carrier completely on or off, as opposed to linear amplitude modulation. Consequently, the audio will be distorted.” He raises a very good point that I hadn’t given much or any thought to. You aren’t modulating signals like they normally would be modulated. The audio quality will be greatly distorted because of this lack of having “linear amplitude modulation” – kr.baker
YOU AREN’T TRANSMITTING AM RADIO FREQUENCIES
You are transmitting at a low frequency that can be heard at higher AM frequencies. Lets say I transmit music at a base frequency of 300 Kilohertz (KHz) This music can be heard at the frequencies of 300 KHz, 600 KHz, 900 KHz, 1200 KHz,… (etc.) This is called harmonics. When the radio receiver’s picks up a 300 KHz signal on a 2400 KHz band, the signal is heard only faintly. If you were to pick the very same 300 KHz signal on the 600KHz band, it would be exponentially stronger. This is why harmonics are only useful to a degree.
Modify your radio
I want you to modify your radio and post a comment below that tells us what you did and how it worked! it’s that simple!
Although, I do have some suggestions:
Try changing R2 to a 3.3Kohm resistor.
Try cutting C3 out of your circuit.
try connecting the radio antenna to ground through a 1Kohm resistor
The radio transmitter we made can only transmit at frequencies from 110KHz to 480KHz. The AM radio band is from 520KHz to 1610KHz. Harmonics are essential to be able to hear audio signals transmitted from our radio transmitter.
Simple FM Radio Receiver
This FM radio receiver circuit is very simple to build and is powered by just a single 1.5V battery cell. Receiver consists of a regenerative rf stage, TR1, followed by a two of three-stage audio amplifier, TR2 to TR4. In some areas 3 stages of audio amplification may not be necessary, in which case TR3 and its associated components can be omitted and the free end of capacitor C5 connected to the collector of TR2. The critical part of the fm radio receiver is the first stage, TR1/VC1, where the wirings must be kept as short as possible. Coil L1 is formed by winding 8 turns of 1mm (20 swg) enamelled copper wire on a 6 mm diameter former, which is then removed. After that L1 should be stretched carefully and evenly to a length of about 13mm.
The tunning capacitor VC1 is one of the two fm sections of a miniature fm transistor radio with built-in trimmers (VC2). The “earthy” end (moving vanes and spindle) is connected to the 22pF capacitor C1. The value of the rf choke L2 is not critical, anything from 1µH to 10µH being suitable. The output is suitable for ordinary earphones connected in series to provide an impedance of 64Ω.
Tuning-in the fm radio receiver
To operate the receiver, potentiometer VR1 must first be advanced slowly (towards the end of the track connected to battery positive) until, at about the half-way point, a sudden slight increase in background noise will be heard, indicating the onset of oscillation. It then should be backed off, very slowly, until oscillation just stops; it then should be possible to tune in some stations.
The correct frequency range of 87 MHz to 108 MHz can be obtained by adjusting VC2 at the high frequency (108 MHz) and slightly stretching or squeezing together the turns of coil L1 at the end (87 MHz).
FM Radio Circuit Diagram
TR1 = BF199
TR2 = TR3 = TR4 = BC547
FM Broadcast Receiver
Here’s a portable FM broadcast radio receiver for reception of FM broadcast band based around FET transistor. The topology is a classic grounded-gate FET VHF Hartley oscillator. The drain resonator inductance is centre-tapped with feedback to the source through a small capacitance. By tapping down towards the cold-end of the coil the feedback isn’t as critical as your usual source-drain capacitor feedback and it tends to be far less difficult to get to work across a broad range of frequencies. The RFC to an RC source circuit to implement self-quenching is very traditional for super-regenerative detectors. The quench gets frequency-modulated somewhat by the drain current, so it varies with signal strength and the recovered modulation, this is typical for self-quenched circuits.
The detector alone provides sufficient audio to drive a crystal ear piece in a very quiet room, giving a true “single transistor” FM receiver. A largish resistor (~10 k) prevents the source circuit from seeing too much of the fairly large capacitance of the piezo element (about 14 nF) and pulling the quench well down into the audio range. Some additional audio volume can be achieved
by redesigning the quench circuit to utilize the piezo capacitance directly, but the source resistance has to be dropped quite a lot to achieve a viable quench frequency and the gain in sensitivity isn’t as fantastic as one might hope. Still, give it a try, a single active device FM radio, pulling < 100 uA is mighty impressive!
The detector can operate with the source resistance approaching 1 M, even at extremely feeble currents it is still very sensitive. Best over-all performance was achieved with 10 K and 6.8 nF in the source circuit.
I decided to add a stage of audio gain, retaining the use of a high impedance ear piece to keep the current consumption as small as possible. I picked a super-beta transistor, the MPSA18, for the audio amplifier, and used a simple self-bias topology. This was all to keep the total receiver current consumption very small and maximise the battery life. The audio quality is quite acceptable (the usual super-regen’ slope-detection distortion and quench inter-modulation with stereo sub-carrier, etc). There is no volume control, the super-regenerative receiver has an AGC-like quality because of its physics. The audio power available is on the low side, it is for quiet environment listening only; not exactly library-quiet, but not the local pub on Friday evening either!
The complete receiver pulls around 500 uA from 6 volts. Four of your average bargain-store dry cells should run the receiver for at least a month continuously. Band-name alkaline cells might run it for a very long time indeed.
Tuning is achieved with a small alignment screwdriver, or similar insulated tool. The trimmer rotor is “grounded”, but hand-capacitance is still slightly present because of the very high frequencies and gains involved (i.e. minor circuit layout strays).
Some effort was put into setting up the trimmer bandspread to cover the FM broadcast band (i.e. picking C5 and C6 to make C7 tune 88-108 MHz). I spent a lot of time doing the algebra to try to come up with a way to calculate the circuit stray capacitance and the actual tank inductance by trial frequency measurements with different fixed capacitances. The solution is truly horrible; involving finding a parabola that fits three points, which means solving a determinate of a 4×4 matrix equated to zero… I gave up after a few hours of wading through my sign and subscript mistakes, the whole experience leaving me feeling somewhat defeated!
I really wanted to achieve a result I could use to write a calculator, not unlike the VFO helper one which I did the “hard way” with pencil and paper as well. It would be extremely useful to be able to determine stray tank parameters just by measuring the frequency produced after a few capacitor swaps. I’ll revisit this I think. Anyway, the geometry of the coil (7 mm diameter and length) gives about 120 nH using the Wheeler formula, and my inductance meter agrees. Some empirical capacitor swapping and trimmer jig twiddling later I arrived at a bandset (C5) of 10 pF and bandspread (C6) of 22 pF, giving a tuning range of 86-110 MHz, give or take. The stray capacitance that fits this is around 4.5 pF if I’ve done the math right. For comparison, my capacitance meter says the detector drain looks like 21.8 pF, but that is without a drain current, having the inductance disconnected, being measured at AF, etc… I’m happy, it tunes the whole band well.
Component substitution: The J310 is obsolete, I just happen to have a lot of them. Any RF FET should be a suitable replacement. The MPF102 is quite suitable. The MPSA18 could be replaced by any NPN signal transistor if you don’t mind burning a bit more current. I’d recommend a low-noise device with good gain like the BC549C or BC550C. You’ll obviously need to experiment (calculate) new resistor values for the audio stage if you change the transistor, the circuit is not particularly Β independent.
You might like to play with the quench frequency by altering R1 and C1. The selectivity is at a minimum 4 times the quench frequency. Lower quench frequencies become audible and will mix down higher signal components. If you want to place the quench below 15 kHz you’ll need to add much better filtering, perhaps a Sallen Key filter or two. Higher quench frequencies reduce the gain somewhat, so pushing it too high is a bad idea. The FM stereo MPX signal has energy to around 56 kHz, more if there are SCA services. Typically the quench is set around 30 kHz (8 kHz into the lower L-R sideband), but as discussed it will vary with signal strength and the modulation. The quench will tend to mix down the L-R sidebands and/or beat with the pilot tone at 19kHz. The result can be absolutely horrible to listen to, especially when the quench is getting pulled around by the modulation or the L-R sidebands are especially intense (lots of stereo difference content). For purely mono signals the recovered audio can be reasonably high fidelity if you position the slope properly. For AM signals (i.e. The Airband) the receiver is especially affective.
L2 is not especially critical, it is just an RFC to isolate the RF signal at the source from being shunted by the quench oscillation capacitor. Anything that gives > j1 kΩ of reactance should be fine, so 1.6 uH or more is sufficient, perhaps a little less would still work. The 10 pF feedback capacitor is about -j160 Ω at 100 MHz, anything at least 5-10 times larger in magnitude than that should be fine. The RFC specified has about j15 kΩ of reactance. A few turns on a ferrite bead will work, as will an RFC wound on a high-value resistor. Just make sure the inductor’s self-resonant frequency is far above the frequency of interest so it is still inductive. It is difficult to make an inductor too large at VHF that would upset the circuit that isn’t already looking very capacitive.
L1 and the associated C5,C6,C7 capacitors can be changed to put the receiver anywhere you like from high-HF to low-UHF. My particular receiver topped-out at 235 MHz with the 120 nH coil (indicating a stray capacitance of around 4 pF which is in reasonable agreement with the bandspread capacitor calculations), but could go much higher with smaller inductances.
Putting the radio on 10 metres is an interesting idea, it isn’t especially difficult to build a miniature AM transceiver using this as the receiver, if you had enough poles on your TR switch/relay you could use the same transistor for the TX and RX, even the same tank. Similar ideas were explored years ago when frequency stability standards weren’t what they are now. I’ve seen articles describing construction of 2 metre HTs using pairs of nuvistors or acorn tubes with free-running LC oscillators on TX and RX, switching around the cathode circuit to achieve either super-regeneration for RX or plate-modulated smooth oscillation for TX.
VHF FM Aircraft Receiver
VHF FM Aircraft Receiver is a superregenative receiver developed for listening to FM transmitters but also tunes the aircraft band and the top portion of the FM broadcast band. Receives both AM and FM (107mHz to 135 MHz). You can use this receiver with the any FM transmitter. The receiver is amazingly simple using only one transistor for the receiver section and one IC for the audio section.
This circuit is a self-quenching regenerative RF receiver also known as a superregenerative receiver. A superregenerative receiver performs two basic functions. First it feeds back a portion of the received signal from it’s output in phase to its input; and second a super audible quenching oscillator drives the amplifier through the point of oscillation and maximum sensitivity and then quenches the oscillation repeatedly. This keeps the feedback from driving the circuit into self-oscillation and allows the signal to be regenerated over and over again.
In this version of the circuit, both functions are performed by the circuitry associated with Q1. The rest of the circuit, shown to the right of L3 in the schematic, comprise the audio amplification circuit and are centered on the LM386 Audio Amp IC. In this configuration the LM386 is set at a gain of 200 and feeds it’s output to a standard 1/8-inch diameter stereo phone jack. The audio can then be heard by plugging any standard stereo headset into the jack.
The controls consist of C1, C2, R3 and R6. C2 provides the frequency selection. C1 provides fine-tuning, R3 adjusts sensitivity and R6 is the volume control.
Connect a 9-volt radio battery to the battery connector and a stereo headset to the speaker jack. You should hear a hissing noise if your receiver is working. Tune the receiver by adjusting C2 with a non-ferrous tool. C1 can be used to fine-tune the circuit or to shift the tuning range of C2 (increase the capacitance of C1 if you want to receive FM broadcast below 107 MHz). With C1 In its highest capacitance position C2 will tune the receiver through the lower portion of the aircraft band and the top portion of the FM broadcast band. With C1 in its lowest capacitance position C2 will tune the receiver through the aircraft band and a little higher.
C2 is at its highest capacitance when the small dab of solder on the top of C2 is lined up with the middle pin. A good way to set the receiver to receive a signal from a 108 MHz transmitter like the FM108 tracking transmitter is to set C2 slightly to the right of its highest capacitance and then tune C1 until you hear the signal from the 108MHz transmitter.
Listening to the aircraft band
You may have to tune around a bit and listen for a while before hearing any aircraft transmissions. Pilot communications are generally short and to the point. Transmission is limited to a few seconds. Since VHF communications are “line-of-sight” you will be able to hear aircraft at 30,000 feet a100 miles away or more but may not be able to hear the control tower that is only 10 or 20 miles away if your view is obstructed by buildings or hilly terrain..
One thing to watch out for is strong local signals. The received signal may be garbled if the signal is too strong. This is usually the case when tuning FM broadcast band stations. To remedy this problem turn the sensitivity to its lowest point and put the antenna in the down position.
Radio Tracking the FM108 transmitter
Hold the radio with the antenna fully extended against the front of your body shielding the antenna with your body. Slowly turn while listening to the “beeping” signal from the FM108 transmitter. The beeping will be the loudest when you are facing the transmitter. If the “beeping” is loud when facing all directions then lower the antenna to the point where you can barely hear the signal when facing in one direction. Continue to reduce the length of the antenna as you approach the transmitter.
Why doesn’t the VHF1 receiver use a directional beam or yagi antenna??
A directional beam antenna for this frequency would be to large for field work. Such an antenna would be over four feet wide and would present a considerable problem moving through brush and trees. The relative short telescoping antenna when shielded by the operator’s body works very effectively for direction finding and is relative easy to work through heavy brush and trees. This technique is sometimes referred to as “Body Fade” and it produces a cardioid sensitivity pattern (see picture). The peak null position is exactly 180 degrees opposite the transmitter.
As mentioned above, the operator should take a reading by holding the receiver against their stomach (the edge of the receiver opposite the antenna should be touching the stomach area). The antenna should extend vertically and be about 6 inches in front of the face. Turn slowly listening for the strongest signal (loudest beeping). Move in the direction of the strongest signal (opposite the direction of the weakest signal). Periodically stop and take another reading, adjust the course and continue to work towards the transmitter. Since the “null” point is much narrower than the maximum signal point it may be easier to use the “null” point to establish the most accurate direction to the transmitter. With a little practice a person can become quite efficient in locating the transmitter although the path taken will be somewhat zigzagged.
L1 .12 uH coil
L3 .68 uH coil
L4 .82 uH coil
Air wound; 1.5 turns
on 3/8 form
#26 insulated wire
R1 680 ohm
R3 50K pot (potensio)
R6 5K pot
R7 10 ohm
C1 2 – 5 pF trimmer
C2 2.5-12 pF trimmer
C5,C11 .002 uF
C6 .001 uF
C8 1000 uF
C10 .047 uF
C12 10 uF
C13, C15 0.1 uF
C14 220 uF
Q1 NTE 108 Transistor