Author Topic: Egg timer project help.  (Read 634 times)

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Offline MarkusAndTopic starter

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Egg timer project help.
« on: April 02, 2019, 08:20:29 am »
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!
« Last Edit: April 02, 2019, 08:39:00 am by MarkusAnd »
 

Online Zero999

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Re: Egg timer project help.
« Reply #1 on: April 02, 2019, 11:56:13 am »
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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