A simple wireless device consisting of a radio transmitter and receiver, operating at a frequency of 27-29 MHz. It can be used to light an LED from a distance of 10 meters or to connect a speaker instead of an LED and listen to amplitude-modulated signals. The receiver uses a super-regenerative circuit, ensuring good sensitivity. This is a simple demonstration device. I'll show you how to set it up using an oscilloscope.
The signal from the transmitter goes to the radio receiver's antenna and then to the input circuit L1C4. L2C2R2 is a sawtooth generator with a frequency of approximately 50-70 kHz, which turns the transistor on and off, charging and discharging capacitor C2. This creates "flashes" in the L1C4 circuit, giving the receiver extreme sensitivity. R4C8C7 are a filter that filters the blanking signal while passing the useful low-frequency signal. The whole difficulty of such a radio receiver lies in filtering the blanking signal. To filter it, a good filter is needed. It can be made using an inductor or an active filter using transistors, which will complicate the circuit. Next, the useful signal goes to a two-stage amplifier and a switch that turns the LED on and off.L2 contains 9 turns of 0.4mm wire with a winding diameter of 5mm
The transmitter is built on a single transistor. The modulation signal is fed to the transmitter from a multivibrator built on two transistors. The multivibrator's frequency is approximately 2 kHz. The transmitter is robust and operates normally. L1 contains 9 turns of 0.35 mm wire with a winding diameter of 3 mm. If you grab the antenna during transmission, the transmitter will operate. A ferrite rod changes the transmitter's frequency.
Transmitter setup. Check the signal at the multivibrator transistor collector with an oscilloscope. The signal should resemble a rectangle with a frequency of 1.3-2 kHz.
Next, check the signal on the transmitter antenna. Switch the oscilloscope probe to 10. You will see the transmitter emitting alternating signal pulses. Everything is fine; the transmitter is ready for operation. A 30cm long wire serves as the antenna for both the transmitter and receiver.
Now let's check the radio receiver's operation. Connect the probe to R2C2L2. You should see a sawtooth signal as shown in the photo. Its frequency should be approximately 50-80 kHz, maybe a little less or more, but the main thing is that it should be a sawtooth.
Set the oscilloscope probe to 10 and observe the signal at the receiver's transistor collector. If the transmitter is not turned on, you will see alternating signal pulses without any change. Now turn on the transmitter and use the ferrite core to vary the transmitter frequency until the receiver detects its frequency. Once this happens, you will see the pulses begin to compress and expand in time with the transmitter's multivibrator signal.
Now look at the signal at the filter output, but disconnect capacitor C7. With the transmitter disconnected, you will see an interference signal from the blanking oscillator
Turn on the transmitter and you will see a useful signal from the multivibrator. Next, connect capacitor C7 and observe the signal. The more the blanking signal is suppressed, the better the receiver will perform.
Touch the base of the low-frequency amplifier transistor, and the LED should light.
The biggest challenge you might encounter is where to connect the lower terminal of capacitor C7 on the ground wire. DO NOT connect the capacitor terminal near the radio's ground wire, otherwise the LED will glow constantly. Connect this terminal to the emitter of the amplifier's first transistor, or find this point experimentally.
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