# Help with calculating component values for bistable multivibrator

Discussion in 'The Projects Forum' started by Yuvraj Abhimanyu, Jul 7, 2015.

1. ### Yuvraj Abhimanyu Thread Starter New Member

Jun 10, 2015
15
0
Hey Guys,
I'm trying to build a bistable multivibrator using BC547B. I understand how bistable multivibrator work intutively.
Requirements are:
1) Should trigger form a single source
2) Current Should only be as much as required to drive transistor into saturation and cutoff as this circuit runs on 9v battery.

My questions are:
1) how to select the component values? I mean wat component values affect wat part of the circuit?
2) I'm trying to build a Multivibrator something like this (http://www.electrosmash.com/images/tech/tube-screamer/tube-screamer-toggle-circuit.png), but googling around i came across a circuit like this (http://studyvilla.com/Images/multivb3.jpg),
Now why is there a diode? which one is correct?
3) Can the simulation be done in LT Spice? Because from what i Know, bistable exist because the two transistors are never exactly same!

2. ### AnalogKid Distinguished Member

Aug 1, 2013
4,516
1,246
2. I think the trigger pulse in multivb3 is shown with the wrong polarity. Go with this:

http://www.electronics-tutorials.ws/waveforms/bistable.html

The two diodes steer the incoming pulse to whichever transistor is on by pulling it's base below ground to turn it off.

1. Each 220K resistor and 1 nF capacitor forms a differentiator pulse former to turn the input trigger pulse into a very narrow spike that toggles the circuit. It is important that this spike be over before the input pulse returns to its normal (high) state. A minimum estimate for this is 3 times the R-C time constant. So if the external trigger pulse is 1 ms wide, the RC time constant should be 333 us or less. The values in the schematic come to 220 us, which meets the requirement with extra margin.

Apart from that, the rest of the resistors can be increased if you want. A 2N4401 has a base cutoff current of 100 nA, so the combined base resistance (3.9K plus 47K in the schematic) must allow at least 10 times that value for reliable operation. R = E / I R = (9 V - 0.6V) / 1 uA = 8.4 megohms is the max value. There also is some of the current shunted to GND by the 100K pulldown resistor. Going with a max value of 1 megohm will allow for that. In general for switching circuits, the collector load should be no more than 10% of the base resistor. So the 3.9K becomes 47K and the base resistor becomes 470K. Or something like that.

3. Some bistable latch circuit have the *possibility* of starting up with both transistors in the same state. In the real world this does not happen because as you said no two transistors are identical. But that matters only at startup. After than the minor differences do not matter. Bistable action does not depend on the two transistors being identical. In fact, if one of the two devices is driving an external load like a relay or light, you can mix one small signal transistor and one power transistor in the same circuit.

ak

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