I was looking at an old schematic and I came across this strange circuit for low voltage detection section of a larger circuit (not shown). The operation of circuit is straightforward, Q1 is normally conducting and the voltage divider R2/R3 set a threshold voltage, above which, Q2 conducts. When Q2 is normally conducting, the Q3 is off due to the voltage divider R1/R5. Upon depletion of power source,R2/R3 can no longer provide the base voltage for Q2 and it turns off. The Q3 turns on and discharges C2 to provide a pulse. Does anyone know why Q1 is used in this configuration without the emitter connected to anywhere?

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  • \$\begingroup\$ It might have been used to save having to use an extra part type. When getting PCBs assembled, each part you use adds overhead, so if you can use a kind of transistor you're already using a bunch of, instead of a kind of diode you're not, that saves money. \$\endgroup\$ – immibis Feb 21 '18 at 22:26

Yes it is just a diode.

There are so many clever but inefficient design choices here, I wouldn't spend much time on it.

The person obviously doesn't know how to make a power on reset even with a miswired pot and 3 transistors, one of which is experimental.

Pulse period is about 1x~2x RC or 32 ms

  • \$\begingroup\$ The pot was my mistake, it's corrected now. \$\endgroup\$ – Baphomet Feb 21 '18 at 21:42
  • \$\begingroup\$ Diode are only a penny in volume , there many ways to create a POR. This is very obsolete. \$\endgroup\$ – Sunnyskyguy EE75 Feb 22 '18 at 0:55

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