05-06-2015, 09:28 PM
When the output of IC1 is low, it "silences" IC2. That's because C3 pulled down to ground and is unable to charge, hence IC2 isn't able to oscillate.
When the output of IC1 is high, IC2 does oscillate. The diode is needed to ensure that IC1 is not interfering with the operation of IC2 when in this state - otherwise, with D1 replaced by a short circuit, C3 will be charged rapidly via the 1k resistor R3. With the diode fitted as shown, it becomes open-circuit when IC1 goes high, meaning that IC1 plays no part in the behaviour of IC2.
So the output is a gated square wave. Change the time constants and you'd almost have the BBC "pips"
An obvious question is why didn't they simply connect pin 3 of IC1 to the top of R4? Thus saving the diode and 1k resistor. With the top of R4 being fed by pin 3 rather than the 12V rail, then oscillation can only take place when pin 3 of IC1 is high - which is the same behaviour, but saving 2 components.
Why doesn't it work? Without bread-boarding it, I can't speculate, but I would start by building both oscillators as separate "lumps", and I would omit the power transistor at this stage (just connect to the 'scope). Once both are seen to be oscillating at the expected frequencies, then I'd add the diode and 1k resistor. Then, I'd test my suggestion of dropping the diode and 1k resistor, then return R4 to IC pin 3 rather than +V. Finally, I'd add in the transistor and loudspeaker (and earplugs!) and see what happens.
0.1" plug-in bread-board is really useful - do you have one?
When the output of IC1 is high, IC2 does oscillate. The diode is needed to ensure that IC1 is not interfering with the operation of IC2 when in this state - otherwise, with D1 replaced by a short circuit, C3 will be charged rapidly via the 1k resistor R3. With the diode fitted as shown, it becomes open-circuit when IC1 goes high, meaning that IC1 plays no part in the behaviour of IC2.
So the output is a gated square wave. Change the time constants and you'd almost have the BBC "pips"
An obvious question is why didn't they simply connect pin 3 of IC1 to the top of R4? Thus saving the diode and 1k resistor. With the top of R4 being fed by pin 3 rather than the 12V rail, then oscillation can only take place when pin 3 of IC1 is high - which is the same behaviour, but saving 2 components.
Why doesn't it work? Without bread-boarding it, I can't speculate, but I would start by building both oscillators as separate "lumps", and I would omit the power transistor at this stage (just connect to the 'scope). Once both are seen to be oscillating at the expected frequencies, then I'd add the diode and 1k resistor. Then, I'd test my suggestion of dropping the diode and 1k resistor, then return R4 to IC pin 3 rather than +V. Finally, I'd add in the transistor and loudspeaker (and earplugs!) and see what happens.
0.1" plug-in bread-board is really useful - do you have one?
