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Voltage drop detector

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Dan (gearhead)

New Member
I want to build a circuit or by one where I can check the supply voltage and when it drops below a certain level I want to draw a pin on a pic from high to low.
I don't want it to draw that much current either....any suggestions?

I was thinking of a simple circuit with a voltage divider and a transistor with the base connected between the two voltage divider resistors and the emitter to the pin and through a third resistor to ground and the collector to the supply voltage but I don't know how it will act on the border line when the transistor opens up through the collector and the emitter..

So any suggestions please post....


Dan (gearhead)

New Member
I can't say that i fully understand that circuit but I only have the supply voltage as a voltage source and the voltage detector can't draw that much current...because the device it is going to be apart of only draws a maximum of 0.66mA....and yep it's a battery low cind of indicator to let me know when to save some data to the eeprom in the pic...


Active Member
The response to a query of my own suggested I use an LM339 comparator. They are low cost and appear simple - I've been tinkering with circuits on a breadboard. Download the datasheet from National. They show several arrangements that ought to work in your application.


New Member
Well that circuit is perhaps abit of overkill for your purposes, since it detects both an undervoltage or overvoltage of the supply line, but comparators are idealy suited for voltage level detection in digital circuits.

Comparator circuits are rather simple to build.
Heres a link that explains how they work and has several circuit examples.


Dan (gearhead)

New Member
I'll look into all of your suggstions... Thanx.

The problem is on one hand that it can't draw too much current and on the other hand that it's a 12V MN21 battery that I'm trying to monitor...

Mechie: I actually read that thread but I was wondering how much amperes the Zener ZD1 will draw when it conducts..a couple of mA....probably too much for me I guess...


New Member
Zener current ?

I've just done some scratch maths and figured that...
My original circuit was to run an LED on low battery (bear with me on this...)
So TR2 collector would need to carry about 20mA for a boring LED
If TR1 and TR2 were BC875 then hfe=1000 (a thousand!)

TR2 collector current = 20mA so base current needs to be 20uA
R1 (base resistor) should be a maimum of 11.3v/20uA = 565k (use 470k).

TR1 will steal this to extinguish the LED so 470k across 12v supply gives 26uA (TR1 collector current)
So TR1 base needs to be at least 26uA/1000 = 26nA (!)

Will a zener diode even start to conduct (bias up properly) at this sort of current ?

It appears that the zener is the critical part here, such low currents will give a voltage lower than its rated value so some 'picking and choosing' will be required to select a device 'on test', but with the 0.6v base-emitter drop in addition to the zener giving the circuit's operating point I would expect maybe a 12v zener giving a 12v (ish) operating point.

The circuit's biggest current consumption is the 470k resistor and (until the LED lights) seems to need less than 30uA.
As you want to drive a PIC input rather than an LED this could be further reduced as the collector load on TR2 need only be 1mA (12k) rather than the 20mA I calculated for...
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