555speakers
I am trying to build an APC using the following schematic:
(source: instructables.com)
And the output is having some issues.
Sound only comes out when the rightmost potentiometer is at 0 resistance, and in that position the rightmost 555 heats up rapidly. In any other spot, the speaker simply makes clicking noises.
One possible difference is that I am using TLC555 chips instead of LM555 as suggested in the diagram. Would that make a difference?
How can I debug ... I see no voltage on pins 2,6 and 7 to ground.
If I understand correctly, those are all fed via R1 so I'd look closely for a dry joint at R1. Even if C1 was shorted, I'd expect twiddling R2 would produce something measurable at pin 7.
If you disconnect power and pull the 555 out of the socket, I think you should probably be able to test c1,c2,r1,r2 in-circuit with a multimeter.
For example I'd expect resistance between socket positions 4 and 7 should be 1K
etc.
Dave Tweed has indicated a simple solution (+1 from me). The circuit for which is...
By charging and discharging the capacitor (C) through the resistance connected to the output (pin 3) the charge and discharge times are the same. By using a fixed resistor (R1) you can set the minimum time for the astable. By adding a variable resistor (VR1) you can easily alter the time without altering the mark/space ratio. The voltage across the capacitor (C) charges and discharges between 1/3 and 2/3rds of the supply voltage set by the internal comparators of the 555. The ON time = OFF time = T = 0.7CR (where R = R1 + VR1) - C in Farads, R in Ohms, T in Seconds.
Best Answer
Using a TLC555 should not be a problem. The behaviour you observe is a failing of the APC circuit itself.
When the rightmost potentiometer is at 0 resistance, the 555 heats up rapidly because, internally, pin 7 shorts to ground during parts of the oscillation, to discharge the 0.1uF capacitor. With the potentiometer at 0 ohms, this results in a direct short across the power rails and a very large current flowing through pin 7 (as well as a direct short discharging the capacitor). The absolute maximum current through pin 7 for the TLC555 is 150mA. To ensure this isn't exceeded, you could try connecting 220 ohm resistors (I'm assuming a 9V supply) in series with both potentiometers, and also between pins 6 and 7 of the rightmost 555.
Most of the audio range of the circuit is contained within the lowest 20% or so of the resistance of the right-most potentiometer. Try reducing the value of this potentiometer, or connecting a 10k 'fine tuning' potentiometer in series with it, if you want more precise control.