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Capacitor Not Fully Charging

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Capacitor Not Fully Charging

Postby ISeeLED » Sun Jul 24, 2011 3:49 pm


I'm coming back to electronics after a few years and have been going through the book "Make: Electronics". I have made the following circuit:


The gray part at the top works fine, the LED blinks on and off periodically as the capacitor charges up and discharges.

The black part at the bottom does not though. The buzzer does not sound at all and no current exits the cathode of the PUT. I checked the voltage at the anode and it seems that the capacitor only charges up to 2.5V then stops, but the voltage at the gate is 2.75V, so I believe the anode voltage is not high enough to allow current through the PUT.

I checked the capacitor in a circuit by itself and it does fully charge, but not in the circuit with the PUT.

I also checked the circuit with the capacitor from the gray part of the circuit diagram (2.2uF as opposed to 0.0047uF) and the buzzer does sound.

Can anyone point out any mistakes I have made or tell me why the buzzer does not sound when the smaller capacitor is used?


Edit: I found the answer on a different forum. I thought a loudspeaker and buzzer were the same thing, so I used a buzzer when I was supposed to use a loudspeaker for this circuit as it's an audio oscillator. The relevant thread is here: http://forum.allaboutcircuits.com/showthread.php?t=57446.
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