| Electronics > Beginners |
| Egg timer project help. |
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| MarkusAnd:
Hello! As a newbie, I thought a nice project for me is to design an egg timer. I was thinking of using two 555-timers. So my plan was to use the second one in monostable mode so that it could be triggered by the output from the first one. As a switch, I'm using the 2n2222a transistor which will connect ground to pin2. So, in short, the first 555timer outputs a signal to the base of the transistor which connects pin2 of the second 555timer to ground. The second timer then outputs a signal to a buzzer. The problem I'm having is that when I'm testing the circuit for the second timer with an LED, the LED is on even though I'm not feeding a voltage to the base of the transistor. In the circuit I have attached I'm using a pushbutton attached to VCC to act as the signal from another timer. What am I doing wrong? //MA EDIT: I figured it out! |
| Zero999:
The LED is reverse biased, so will never light, which I take it is a drawing error. Pin 2 should have a pull-up resistor. It will no doubt work without one, but just letting it float makes the circuit susceptible to false triggering, due to noise pick-up. |
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