# Astable Multivibrator Switch

Discussion in 'The Projects Forum' started by marvelo, Jun 24, 2011.

1. ### marvelo Thread Starter New Member

Jun 24, 2011
1
0
Hello everyone.

I'm attempting to build a variable rate 555 Timer, Astable multivibrator to switch a circuit on and off. I am running into trouble working through the theory here.

A little background. I have acquired some retired professional strobe lights from my theater group. The company no longer has whatever device was used to control them. The connection is a 3 pin XLR. Across pins 1 and 3 there is a voltage of 6vdc give or take. Shorting the two pins together makes a single strobe flash. Therefore I conclude that completing the circuit between the two is the trigger. From there I figure that an astable circuit with a potentiometer to control rate is the best bet to get this thing working.

My brain is breaking over whether it is possible to power the 555 circuit as well as switching the pins from the strobe supply voltage. So far I have got it to strobe super fast but with no control.

Is it possible to make this circuit work or do I need a separate power source for the 555 then switch the pins with a transistor?

I apologize if I am lacking a bit of basic knowledge base please try to help me fill out my understanding.

-Tom

PS, the circuit I'm trying to complete is the at this link. Replace the Buzzer with the strobe (pin 1 = red, pin 3 = black)

http://www.electronics-tutorials.ws/waveforms/tim60.gif

2. ### #12 Expert

Nov 30, 2010
17,840
9,177
The first thing to do is measue the current you can get out of the 6 volt connection without firing the strobe. There are some very stingy circuits that might be able to run off a few milliamps. Do you have some resistors to test this with?

I'm getting nothing out of that link.

http://www.electronics-tutorials.ws/waveforms/555_oscillator.html
maybe this one will work.

ps, while you're at it, find out how much current has to flow to force a strobe event.

Last edited: Jun 24, 2011
3. ### SgtWookie Expert

Jul 17, 2007
22,194
1,761
I don't know how much current it requires to trip your flash, or how much current the flash can source without it tripping.

However, I threw together a schematic & simulation using a CMOS 555 timer and a couple of transistors with supporting components. Refer to the attached.

The 555 timer is powered via D1 from the flash. C4, a 10uF capacitor, charges via D1, and keeps some voltage across the timer when the circuit shorts out the flash pins (I'm also assuming that the flash pin 1 is the negative reference that you used; I'm calling it FlashGnd).

The output of the timer can't be directly connected to the flash pins; it basically needs a switch that opens and closes. I've more or less simulated that using transistor Q1. However, when you use a transistor like that, it inverts the logic output of the timer, which starts off being high - so another transistor, Q2, was needed to invert the logic again. Since the TLC555 can sink more current than it can source, I decided to use a PNP transistor instead of an NPN.

I've added R3, which protects the timer in case you turn the pot all the way down to zero Ohms, and helps to prevent C4 from discharging completely. You can increase R3, but if you decrease it, the circuit may get "stuck".

R2 has been decreased in order to reduce the amount of time that Q1 shorts the flash pins to ~2mS; this also helps to keep C4 from discharging completely.

R4 limits Q2's base current. R5 ensures Q2 is turned off unless the 555 output is low. You might be able to substitute a standard LM555 timer if you add a diode between the junction of R4 and R5, anode towards Q2's base. If you are using a CMOS timer, you can actually omit R5.

R6 limits Q1's base current. R7 keeps Q1 turned off unless Q2 is sourcing current.

As shown, the flash frequency will be adjustable from about 0.9Hz to 30Hz. If you wish to be able to decrease the frequency more, increase the value of the pot.

You may find an audio or log pot to be easier to use than a standard linear pot. You don't need to use all three pot terminals; just the middle terminal and one "end" terminal. If an audio or log pot seems too "touchy", try using the other end terminal.

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