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I came across these two circuits, and tried them out successfully:

LM3914 Battery Monitor, drives 10 LEDs in a linear relation to the input voltage.

NE555 Low-Voltage Cutoff, uses a 555 timer to switch off a circuit at low voltage, with hysteresis to prevent uncontrolled switching.

My question is, is it possible to combine the two? That is, use the LED outputs of the LM3914 to control the low-voltage cutoff behavior of the 555 timer?

EDIT Circuit details:

I have a 12V lead acid battery, I want the circuit to automatically cut off power at 11.5V and restore power above 12.5V. With a bit more research, I think I can directly hook up the appropriate LED pins of the LM3914 to the trigger and reset pins of the 555? Can anyone confirm whether that's right?

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  • \$\begingroup\$ Yes but why that one? \$\endgroup\$ – Sunnyskyguy EE75 Oct 2 '17 at 2:04
  • \$\begingroup\$ @TonyStewart.EEsince'75 Why not? :-) But, I figured it would make a neater circuit if it works, but I'm not entirely sure how to go about it. \$\endgroup\$ – Boloar Oct 2 '17 at 2:09
  • \$\begingroup\$ If you need an LDO and a relay with a 555 ok but LM3914 REF OUT can bias a comparator with feedback ratio from Vbat the same threshold to drive a Pch FET has no moving parts. \$\endgroup\$ – Sunnyskyguy EE75 Oct 2 '17 at 2:17
  • \$\begingroup\$ @TonyStewart.EEsince'75 Could you explain that more clearly? I don't quite follow. I'm not a pro at electronics hahah \$\endgroup\$ – Boloar Oct 2 '17 at 2:20
  • \$\begingroup\$ 1st define Vbat and LED scale then low cutoff \$\endgroup\$ – Sunnyskyguy EE75 Oct 2 '17 at 2:26
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LM3914 Ref Out can feed this. 11.5V is Vbat at cutoff

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ It's gonna be pretty difficult to get 1MOhm resistors where I'm at - I couldn't get 200k or 500k because there's not much demand for it. After a bit more research, I see the 555 chip has an in-built comparator as I understand it so that's how it works so well as a low voltage cutoff. I'll keep trying to get it working with the 555 timer because that's conveniently available for me. Will surely try with the FET if I can find a good one. \$\endgroup\$ – Boloar Oct 2 '17 at 4:56
  • \$\begingroup\$ All R values may be scaled up or down with same ratio. Cars use these FETs everywhere. Use dot mode in LM3914 to conserve power Even 3914 has comparators so choosing lowest or 2nd lowest LED for OFF gate voltage to eliminate part is possible. Battery memory may cause oscillation , so choose more hysteresis \$\endgroup\$ – Sunnyskyguy EE75 Oct 2 '17 at 12:51

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