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enter image description here

This is a pretty old alarm system based on an NPN phototransistor.

Why are there two transistors here, T1 and T2? Can we say that T1 is an emitter follower that amplifies the phototransistor current and T2 is a mere switch used to invert signal? Or, rather, T1 and T2 are a Darlington transistor?

What is the purpose of the R3 22 Ω resistor? If T2 works as a switch, wouldn't it make sense just to ground its emitter?

What is the purpose of C1? Are C1 and R1 an RC filter? What is R6 for? Sort of a fuse?

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    \$\begingroup\$ What psycho drew that circuit upside-down ?!? \$\endgroup\$
    – brhans
    Jan 30, 2023 at 22:35
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    \$\begingroup\$ @brhans There is a special place in hell reserved for them. \$\endgroup\$
    – winny
    Jan 30, 2023 at 22:51
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    \$\begingroup\$ It actually makes some sense when dealing with mostly PNP devices. \$\endgroup\$
    – PStechPaul
    Jan 31, 2023 at 0:27
  • \$\begingroup\$ @PStechPaul But they are flipped PNP using positive rail. \$\endgroup\$ Jan 31, 2023 at 14:39
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    \$\begingroup\$ It's not upside down. It uses PNP transistors and positive rail is the GND for them. \$\endgroup\$ Jan 31, 2023 at 17:49

3 Answers 3

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Schematic with PNP's is sometimes not easy to "decode" ...

So, simulate it and check all that is needed for knowing the behavior.

Here are three configurations of that "thing".

Working mode: enter image description here

Working mode, relay test: enter image description here

Working mode, IR beam stopped, relay ON, buzzer ON (not showed). enter image description here

DC Analysis showing the different "areas" of behavior (3 or 4) ...
And also the changes with the values of R7 (current of Q3 & Q4).

enter image description here

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  • \$\begingroup\$ Thanks for doing this for me, but I don't see how it's helpful. It doesn't answer any of my questions. Which are: does the Q1 work as an emitter follower or not? Does the Q2 work as a switch? What is the purpose of R2 and R7? Like, if you remove the R2/R7 from your simulation, it will work just the same. \$\endgroup\$ Jan 31, 2023 at 17:58
  • \$\begingroup\$ Does the Q1 work as an emitter follower or not? Yes. Does the Q2 work as a switch? No. R2 work as feedback resistors which reduce the "gain" of Q2 and R7 reduce the gain of the Darlington Q3 and Q4. To see this, one must measure voltages and currents versus the input, in that case i(R1). In fact, one must search the limits of voltages and currents. We should then find that there is a "logic" state, followed by a "linear state, and then another "logic" state. This is found when doing an "DC analysis". I will add in answer because this is important for delimiting the "regions" of behavior. \$\endgroup\$
    – Antonio51
    Jan 31, 2023 at 18:17
  • \$\begingroup\$ See the last picture which is a "DC Analysis" showing the "3" areas of behavior. Two "could" be considered as a "logic" area, but not the "center" region. \$\endgroup\$
    – Antonio51
    Jan 31, 2023 at 18:49
  • \$\begingroup\$ I just realized there is no resistor in series with Q3 base. Is this why they put the 68 Ohm resistor at the emitter? To limit base-emitter current? \$\endgroup\$ Jan 31, 2023 at 20:47
  • \$\begingroup\$ Yes, as the DC Analysis shows, the current Ic from T3 & T4 is somewhat limited by R7. But note also that current is not very well limited through SP201- T1/BE-T2/BE-22 Ohm. \$\endgroup\$
    – Antonio51
    Feb 1, 2023 at 6:23
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While T1 and T2 do not form a true Darlington pair, in many ways the arrangement is better. You get the same high current gain. But because the T1 collector is not connected to the T2 collector, T2 can saturate. This can pull the D2 cathode up (toward +U) high enough to cut off T3 and T4 (a true Darlington pair).

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  • \$\begingroup\$ Can we say that T1 works as an emitter follower that amplifies phototransistor signal? And what is the purpose of the R3 resistor? \$\endgroup\$ Jan 30, 2023 at 22:44
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T1 and T2 are a direct coupled amplifier, not a Darlington pair. Their output goes through D2 to drive T3 and T4 which are a Darlington pair used to switch the relay.

C1 is probably for RF immunity or to slow down the response so it doesn’t chatter on and off,and R1 is for biasing T2.

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  • \$\begingroup\$ C1 is obviously a low pass filter because it goes from the base to the ground. However, the biasing resistor R1 is connected between base and emitter, and not to the ground. Why is that? It just gets more and more confusing... \$\endgroup\$ Jan 31, 2023 at 16:21

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