Activation of 555 timer (astable) with PIR

[Daryl Musashi]

Hello,

I want to activate a 555 timer (astable mode) with a PIR. The PIR sends a positive signal for a certain time, when motion is detected - how can I use this signal to activate the 555 timer?

The astable 555 timer is already built, but starts working immediately when the circuit is powered. Thanks for assistance.

 

[DerStrom8]

Do you want the astable timer to keep running indefinitely? Or only for a period of time?

Triggering it shouldn't be a problem. You'll probably want a second 555 wired in monostable mode, with the trigger (pin 2) connected to the collector of an NPN transistor. The emitter should be connected to ground, and the base connected to your PIR sensor output. When the PIR sends a positive output, it switches the transistor which triggers the monostable 555. The output of that can then be used to power your second 555.

There are quite a few different options, it would help if we knew exactly what you were trying to do.

Regards,
Matt

 

[alec_t]

...and the base connected to your PIR sensor output.

via a resistor (e.g. 10k).

 

[DerStrom8]

via a resistor (e.g. 10k).

Woops, forgot to mention that. Yes, you'll need a base resistor to limit the current, otherwise you could fry your transistor.

 

[cowboybob]

Don't forget a (10K or so) pull-up resistor between Vcc and pin 2 of the 555.

Such a component is implied but not specifically mentioned.

 

[DerStrom8]

Don't forget a (10K or so) pull-up resistor between Vcc and pin 2 of the 555.

Such a component is implied but not specifically mentioned.

Another good point. Thanks guys.

 

[Daryl Musashi]

Your hint using a monostable 555 timer and a NPN-transistor worked to power the second astable 555 timer - thanks to all of you for your help.

My project contains building a birthday bomb (dummy), which is activated by a PIR-sensor. It counts down from a specified value (21 in my case). When "00" is reached the counting stops and a birthday music is activated (by using a UM66T08S semiconductor and a speaker). The part with the music/speaker is not implemented yet in reality (but purple at the circuit diagram) - I am still waiting for the UM66T08S to be delivered.

Furthermore finally a little motor will be triggered at the end of the countdown, which releases a little rod on a spring. At the rod a little flag with "Happy Birthday" or something like that will be written. In general, the triggering of the motor should work in the same way as discussed in this thread I think (transistor + monostable 555). The attached circuit diagram does not contain the part with the motor, yet.

At last I apologize for the poor circuit layout - it is still under development and this is my first project . I marked the "important" (in my opinion) wires red. It was created with TinyCAD.

___

Actually all components will be powered by a 9V battery and a thyristor in order to reduce the voltage to 5V.

 

[cowboybob]

How long are you expecting the PIR, once triggered, to stay "on" before turning "off"?

How are you expecting the circuit to respond to a second, unexpected PIR pulse before the remainder of the circuit has completed the count-down?

 

[Daryl Musashi]

The PIR itself is active as long as the battery is connected. With C1 and R1 (at the monostable 555 timer) the duration is set to power the whole circuit. In my configuration (47 µF, 690k Ohm) the circuit is powered for 35 seconds.

A second, unexpected PIR pulse during this time has no effect.

When the counter reaches "00" the reset pin 4 of the astable 555 timer is set permanently low. Therefore, a second PIR pulse after the counter reached "00" has no real effect, because the circuit is unable to count down in this state. Only the seven segment displays are powered again and show 00 for again 35 seconds (which is not really desireable I must admit ).

To avoid powering of the seven segment displays at the final state I must modify the powering of the displays slightly, thank you.