simulate this circuit – Schematic created using CircuitLab


I have a low battery detection circuit here. When the voltage of the battery goes below 10.5, I have an LED flashing (Uses the NPN transistor and the I/O "1"). This part of the circuit works great. But I also have a shutoff circuit in place where if the voltage of the battery continues to fall and reaches a 2nd threshold, the relay that has BATT and BOUT included in it opens. My first prototype worked well but I was using a relay with a coil voltage rating of 5V (The voltage of CPNP could be as high as 11 V when the battery is at good health). That relay was getting hot.

So I replace that relay with one that is rated up to 12V. But now the system isn't quite working the way it should. If the voltage of BATT is good, the relay turns on like it should, but it will not turn off at the threshold I have set. I noticed when the threshold is met, the collector (CPNP) voltage drops, but still above on 1 or 2 volts which is still enough to keep the relay on. I have added a flydiode to the relay coil as well as placed a 47k ohm resistor on the emitter and base of my PNP resistor but I still get the same result.

Should have I Used a FET instead for this? Is it possible before I place the flyback and the 47 kohm resistor, that I damaged the transistor? I notice when I have OUTPUT 2 set to be low (LM358 <1 volt), there is a still a significant voltage on the PNP side of that 4.7 kohm resistor (between 0.6 to 0.7 away less then emitter). So I think the main goal is to fix the issue with CPNP voltage, how can I make that get closer to zero?

  • 2
    \$\begingroup\$ Use 2 x <Enter> to insert paragraph breaks to make your post more legible. The wall of text is a bit offputting. \$\endgroup\$
    – Transistor
    Commented May 12, 2019 at 20:26
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    \$\begingroup\$ You really need to use the schematic editor in your question. The wiring diagram (kind of) you show is very difficult to read and requires putting a finger here, while looking over there, etc. Too easy to make mistakes for some of us (genius wiring diagram readers excluded, of course.) \$\endgroup\$
    – jonk
    Commented May 12, 2019 at 20:26
  • \$\begingroup\$ Thanks jonk, new to stack exchange. I hope this is a better representation of the circuit. \$\endgroup\$
    – PhippsTech
    Commented May 12, 2019 at 21:39
  • \$\begingroup\$ What is the U3 device? \$\endgroup\$ Commented May 13, 2019 at 0:05
  • \$\begingroup\$ You have no hysteresis in your circuit, and as the current varies through D5 the voltage into your detection circuit will vary. It is unlikely to be stable. \$\endgroup\$ Commented May 13, 2019 at 0:07

1 Answer 1


The main issue in this circuit was the maximum voltage that was being applied to the base of the transistor wasn't enough to compared to the emitter. The voltage divider circuit seem to do the trick but it was also decided that I could put a p channel MOSFET in the place of the PNP transistor and the relay and the circuit has been performing great.


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