Simple Radio Beacon with tone modulation
Following on from
Simple Radio Beacon :
An additional modulation stage is added which impresses a 600Hz tone on the carrier.
The only audio oscillator that will run on a (low) 1.5V supply
seems to be: the cross-coupled (Eccles-Jordan) multivibrator type.
Must be constructed with discrete components.
No IC's (555, Op-Amp, TTL, CMOS etc) will run at 1.5Volt
Output is a "square wave" With circuit values shown freq' is approx 600Hz. (see formula) Amplitude of waveform is; almost zero to near rail voltage (1.5V)! A 1000 pF (1nF) coupling capacitor modulates the base of the RF oscillator. With modulation: the pulsing rate of RF oscillator increased a little. Modulator DC current drain measured at 2mA (3mW) (same as RF osc)
Beacon reception will now be an on-off tone which is more audible than the thump-thump of an unmodulated AM carrier, as heard on an "AM detector" type receiver. Modulated Beacon Off-air audio sample (AM 'envelope' detector) For comparison: Sputnik off air beep c1957