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[SOLVED] 555n timer. How to control VCC from other 555n's output?

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Newbie level 5
Jan 10, 2013
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Hello, it's nice to have found this forum! I've been messing for a month with electronics and try to create some kind of a small alarm, just to practice.
I have attached a png of the circuit i have created so far. A NOR gate takes input of at least one of two buttons, giving LOW output to the first 555n. This triggers and gives HIGH for some time ( depending on the potentiometers value ).
What i want is to take the HIGH output of this first 555n and control the VCC of a second. The second 555n gives a high frequency making a piezo speaker buzz. The problem is that i don't know how to make the first 555n's output work as a switch for the power supply of the second 555n. I have tried using a 2222 transistor but had no luck. As far as i know the transistor except from working as a switch it increases the current on the emitter, so it could ( maybe ) destroy the second 555n.
Well, i've been very confused at this point.:sad: Do you have any idea how can i fix it? I have attached the circuit in png ( designed with Geda ). Thanks.
( sorry if the design of the circuit is a bit confusing, it the first time i design an electronic circuit ).


  • alarmProject3.png
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Why not connect it to the reset pin of the second 555? You can then leave it powered up and use the reset signal to start/stop the buzz.


betwixt you are probably right. Well, i have to read some more stuff about the 555n. When i directly connect it to the reset pin it doesn't work. I'll come back when i'll have some progress. Thanks for the help!

That's a risky way to do it. Depending on the remaining circuitry, the load on the VCC can be 200mA or more.


Well, the plans have changed. I'll throw the output to an led just to light as the cycle completes. What i saw is that the reset pin stays at 0.8 V and stops the timer when it goes below. So i plan to take a high output from my arduino ( it will give high when someone gives the right password ) and use it to drop somehow the 0.8 V reset pin down to 0 ( close the "alarm" if passwd is correct ). That's where i'm stack now. How to take a HIGH digital output and someway cause a 555n reset timer go below 0.8 V. I tried with some n2222 transistor but couldn't do it. I tried with PNP transistors too.
Do you have any idea about this too? Thanks.

You could use a MOSFET instead of bipolar transistors. There are many of them with very low resistance in the mOhm range so you won't have the V drop associated with biploar devices between the emitter collector.

In this case a MOSFET would effectively ground your reset pin.

I'm no expert in Arduino but I assume it has a normal logic output and that's what you are using. Why not connect it directly to the reset pin or via a resistor of say 470 Ohms. The current from the reset pin is very low so it should easily be pulled to 0V by a logic output.


Unfortunately i don't have a MOSFET transistor. But did it another way. I connected the arduino pin to an N4148 diode ( for security ) and then to a 1K resistor and then to the positive leg of the 470μ capacitor. So when arduino pin gives HIGH it teaks the capacitor and stops the cycle. However I can't understand how the trick is done, as i'm new to electronics but at least it works!
Thank you all for your help!
Betwixt i will try the way you suggest for curiosity:wink:

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