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

[saavedra29]

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 ).

 

[betwixt]

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.

Brian.

 

[saavedra29]

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!

 

[Magvitron]

use some thing like this, the out put is disabled normally . when a high pulse is given; the 555 is activated.

 

[betwixt]

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

Brian.

 

[saavedra29]

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.

 

[ads_ee]

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.

 

[betwixt]

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.

Brian.

 

[saavedra29]

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: