AM Radio built around LM555


Posted on Feb 4, 2014

AM radio built around 555 timer chip. The only active device (silicon, germanium, or otherwise) is the LM555. The tuning is accomplished with an inductor and a capacitor, and the LM555 acts as an AM demodulator and class-D power amplifier to drive the speaker. You may be wondering how all this is accomplished with a 555. Here`s how the circuit wor


AM Radio built around LM555
Click here to download the full size of the above Circuit.

ks: The AM radio signal is tuned by inductor L, which is 300 turns of wire on a 1/2 inch diameter cardboard tube made out of a paper roll, along with the 100pF variable capacitor. One end of the parallel configuration of L and C connects to an antenna (surprisingly long!) and the other end connects to a ground wire which is tied to the AC outlet ground (old books tell you to ground it to a water pipe). So far this is exactly like an AM crystal radio. The 555 timer is configured as a pulse width modulator in a non-traditional configuration. If I used the standard approach and connected the input to the CV pin, the low impedance of the pin would prevent the circuit from receiving any radio signals. I had to invert the circuit and tie both high impedance analog pins, Threshold and Trigger to the radio signal input. This is the reason why the CMOS version of the 555 timer performs much better than the standard bipolar, which has higher input bias current. The pulse width modulator ramp is created by the 0. 01uF capacitor and the 10K bias potentiometer which are connected to the Discharge pin. The potentiometer wiper goes to the LC arrangement. With no radio signal coming in, the voltage on Threshold/Trigger ramps up until it hits the threshold, and then Discharge causes the voltage to ramp down again. When a radio signal comes in, it gets superimposed on the ramp signal, causing the threshold and trigger comparators to trip...




Leave Comment

characters left:

Related Circuits

  • New Circuits

    .

     


    Popular Circuits

    Fet-probe
    Infrared Remote Controller
    LED Color Fade Effect circuit diagram
    Simple TV Transmitter circuit diagram (VHF)
    USB powered device with multiple Decoupling Capacitors
    40 MHz Wideband RF Amplifier
    Door Lock on Go
    DCF77 Preamplifier
    Huffman-coded Audio
    Single-speed motor excitation from the electrical braking circuit
    Use TWH8751 street light control circuit



    Top