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My project requires the use of a NE555, which turns on and starts operating when a 3-position switch is either up or down (not in neutral).

To do this, I am using an OR-gate (no problems here) and a 2N3906 PNP transistor (been using only NPN in projects up until now).

To trigger it, I have come up with the following circuit in Multisim 14:

Design V1

This way, when the gate output is low, the transistor's base is floating, when the output is high, the base is working as a """voltage divider""". I chose to use a 4N26 to not worry about the NPN calculations.

As far as I can understand, I am forcing a saturated state, focusing on Vce reaching 0. I have been changing my base resistors values to achieve this, haven't done any additional math.

Is my design all right? Will it perform well when powering the timer that, say, requires 15mA? I am willing to learn!

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    \$\begingroup\$ Why do you need all this - OR gate, optoisolator that is not wired to isolate, and yet a transitor for something? Why can't the switch just give 5V power to NE555 directly when in non-center? \$\endgroup\$ Commented yesterday

3 Answers 3

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You've got the PNP transistor E & C reversed. It will actually function in that configuration, however the gain will be quite low, maybe 10 or so rather than a couple hundred.

Other than that it looks functional. The optoisolator adds nothing functionally if the grounds are common and it has a "gain" of only 0.2 so it's rather a liability. You should replace it with an NPN transistor and move the resistor, or (even better) use a NOR gate and drive the PNP base directly through a single resistor.

Far from simplifying calculations, optoisolators introduce a whole new set of concerns such as aging and the wide variation and low current transfer ratio (CTR).

This is a decent value for the base resistor. I've used a forced beta of 20, meaning the base current should be 1/20 of the collector current. This is using your number for the load current of 15mA. If that number is different, the base resistor can be recalculated.

The 'on' base current is about (5V - Vbe)/5.6kΩ \$\approx\$ 0.75mA

schematic

simulate this circuit – Schematic created using CircuitLab

Note that the CMOS NOR gate I've assumed has push-pull drive so there is no need for a resistor to deal with leakage as you'd want if you were using an NPN transistor to drive the base.

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  • \$\begingroup\$ Hey Spehro, first of all, thanks for your tips! I have removed the optoisolator, changed the OR to a NOR gate, and added one resistor in its output connected to the PNP base. (flipped the E and C as well, first image was wrong). Now, Ib is around 20uA, and Ic is around 900uA (but kind of oscillates between 300 and 930). The Timer is working despite the low current, but the simulation crashes some seconds after turning the timer on. I am trying with Rb values above 100k, should I keep using trial and error or choose the value by formula? \$\endgroup\$ Commented yesterday
  • \$\begingroup\$ See edit for base resistor value. \$\endgroup\$ Commented 20 hours ago
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Your circuit seems overly complicated.
Here's a simplified circuit that should do what you want:
It uses the Q1 transistor input to give a wired OR function.

enter image description here

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The fact that you share the same ground between systems on both sides of the opto-isolator, renders that opto-isolator useless. Also you don't need an OR gate, a couple of diodes will do:

schematic

simulate this circuit – Schematic created using CircuitLab

If either switch SW1 or SW2 is closed, one the diodes will be forward biased, conducting current into Q1's base, switching it on. That pulls Q2's base low (via R4, to limit Q1 base current), switching that on too.

R1 and R3 are just to divert any stray or leakage currents (while Q1 and Q2 are supposed to be off) around their bases, to keep them firmly off.

Strictly speaking, you could dispense with D1 and D2, connecting the switches directly to R1/R2, as Carl suggested, but I opted to include them, since I don't know what else you intend to connect to those switches. This answer is really just to illustrate a "diode-OR" alternative.

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