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I am trying to understand how the PNP transistor configuration in this circuit works:

Enter image description here

It comes from this article: How to build a Simple IR Transmitter and Receiver Circuit using 555 Timer?

I believe all the articles I have read about using PNP as switch would have the emitter connected to the positive supply with the load between collector and ground. However, as given it seems to work, but I don't think I understand why. (Note I am using TSOP4838.)

I would ultimately like to use the PNP transistor to replace the switch in this circuit:

555 Bistable

Which comes from this article:

Bistable Multivibrator Using 555 Timer

The goal is to receive a pulse of IR and switch the LED between on and off in response to each pulse.

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3 Answers 3

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Your circuit works as drawn but the base-emitter is reverse polarized (instead of forward). The transistor will still work but with significantly smaller amplification.

As for your intended circuit, I think you could just stick with an NPN as indicated in the 555 schematic, but you could also use a PNP transistor while connecting its emitter to the V+ supply side and the bulb between 0V and the collector.

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  • \$\begingroup\$ I can confirm the transistor is connected as in the schematic. I will check the supply voltage. Otherwise I will just work on the standard config of the PNP. Thanks for answer. \$\endgroup\$
    – RickyBoy
    Commented Apr 15, 2023 at 12:24
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    \$\begingroup\$ With the collector and emitter swapped, the transistor can still function just fine in reverse-active mode, with a much reduced gain. There are occasionally reasons to use a BJT in reverse active, but they're not very common--it's more likely a mistake, in the case of the schematic in the question. \$\endgroup\$
    – Hearth
    Commented Apr 15, 2023 at 15:04
  • \$\begingroup\$ Supply voltage was not the problem so I am assuming (given another answer put forward) that your first suggestion is the explanation. Thank you. Back in business. \$\endgroup\$
    – RickyBoy
    Commented Apr 15, 2023 at 18:42
  • \$\begingroup\$ @Hearth You're right. It was one of the details I have learned years ago but forgot. \$\endgroup\$
    – Edin Fifić
    Commented Apr 16, 2023 at 20:11
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The circuit is wrong, but it may still be able to limp along.

A transistor will show transistor action when connected in reverse like that -- meaning that the "official" base-emitter junction will be reverse-biased, and will conduct when the "official" base-collector junction forward biases. But the beta will be dreadfully low.

I don't know the details -- I'm just remembering this as a tidbit from a class on TTL circuits decades ago, where this action is used to advantage in a typical TTL gate.

It could be that your transmitter circuit is working, but very poorly. If you have an oscilloscope you may want to look at the hot end of that 470 ohm resistor and see how high it's getting pulled -- it should be getting pulled up to 4.8V or so -- if it's not, then yes, reinstalling the transistor "correctly" should give you a lot more light output.

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    \$\begingroup\$ Your description is correct... I would characterise the circut as "limping along" as drawn. I turned the PNP round to what I thought it should be and the response is correct and much more responsive. The transmitter is working no problem (in fact TV remote just as effective transmitter). Judging by answers provided the circuit as published is wrong. Thank you. \$\endgroup\$
    – RickyBoy
    Commented Apr 15, 2023 at 18:39
  • \$\begingroup\$ Yes the pnp is upside down, but I wonder if it's done on purpose to reduce the sensitivity of the receiver, so that it will require a long push on the remote to turn the output high. Looks like a "let's see what happens if I mix components in this way" design philosophy. \$\endgroup\$
    – Sredni Vashtar
    Commented Apr 16, 2023 at 16:49
  • \$\begingroup\$ Well, OK -- but if that's the intent then it should have been done by installing the PNP correctly and increasing R2 until the desired results were achieved. Even when you're designing a circuit to use a transistor in its intended mode you don't count on things like \$\mathrm{H_{FE}}\$ to be constant from part to part. \$\endgroup\$
    – TimWescott
    Commented Apr 16, 2023 at 17:05
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    \$\begingroup\$ I have seen circuits on YouTube that are certainly not the result of any reasoning. "100W amplifiers" that are just glorified distortion generators, switching circuits whose interferences can be picked up on Mars, transformers with a common terminal between input and output, that in some systems might be the live mains wires... My guess is the 'designer' put the transistor backwards by error, noted the circuit worked as expected and since when hooked up right it was too sensostive, called it a day and copied the new schematic. It's just a guess... :-) \$\endgroup\$
    – Sredni Vashtar
    Commented Apr 16, 2023 at 18:38
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The circuit is wrong. The collector and emitter are rotated.

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    \$\begingroup\$ That's what I thought. But it seems to work. Is it possible that some BC557 the pinout is different to others? Or is it standard? All pinouts I have found so far are the same. \$\endgroup\$
    – RickyBoy
    Commented Apr 15, 2023 at 12:08
  • \$\begingroup\$ It is a standard and it is defined in the datasheet. \$\endgroup\$
    – user337103
    Commented Apr 15, 2023 at 12:13
  • \$\begingroup\$ Yes I looked up datasheet and it is as expected. Circuit works though, as in the diagram. \$\endgroup\$
    – RickyBoy
    Commented Apr 15, 2023 at 12:20

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