# What is th simplest way to monitor a voltage in a circuit?

Discussion in 'General Electronics Discussion' started by JPU, Jan 6, 2016.

1. ### JPU

281
1
May 19, 2012
Hi All.

I have a 5V simple picaxe circuit. I simply want to use the circuit to monitor a voltage of a secondary circuit which is about 10V. If the voltage of the secondary circuit goes above 20V, I then want the first circuit to detect this voltage and then turn on a warning LED.

I would like the circuit to be as simple as possible. Is there a simple way I could do this e.g could I use a voltage supervisor?

JPU

2. ### BobK

7,682
1,688
Jan 5, 2010
Hopefully, the two circuits have a common ground. Otherwise it gets more complicated.

Assuming a common ground:

You could use 2 resistors to make a voltage divider and feed it directly into a digital input. I would also use a 5.1V Zener to clamp the input to the PICAXE.

Use a Schmitt trigger input and look up in the datasheet what it's low-to-high threshold is. Then make the voltage divider output that voltage with 20V in.

On the 12F1840, my favorite little PIC, Schmitt trigger inputs have a threshold of 0.8 Vcc, so, 4V for you 5V PIC. So you would use a divider that outputs 1/5 of the input or a ratio of 1 : 4 for the bottom and top resistor. 10K and 40K for example. Put the 5.1V Zener across the bottom resistor, anode to ground and cathode at the junction of the two resistors.

Bob

3. ### BobK

7,682
1,688
Jan 5, 2010
The words might have been confusing, so:

Bob

4. ### BobK

7,682
1,688
Jan 5, 2010
Actually, the Schmitt trigger input is a bad idea. It will not reset until the voltage falls quite a bit.

Use an analog input. This allows you to set the threshold anywhere you want.

The same circuit applies.

Bob

5. ### JPU

281
1
May 19, 2012
Hi Bob

Thanks for helping with this, However I am very very novice. Please can you explain what you mean by "use an analog circuit".

In your schematic that you have kindly drawn should the "out" connect directly to the PICACE input or does this need to connect to something else first?

Sorry for sounding novice, despite the number of posts I have made and the VIP status I have; they were all questions!!

Thanks

JPU

6. ### BobK

7,682
1,688
Jan 5, 2010
The "out" of my circuit connects directly to a PIC pin which is an analog input. You use the A/D converter in PIC to read the voltage on it. Anything over 4V would indicate the input voltage is over 20V.

When using the A/D converter, you should add a 0.1uF capacitor in parallel with the Zener diode. This will stabilize the input voltage and remove noise.

Bob

7. ### Bluejets

5,247
1,099
Oct 5, 2014
Bob,
Just wondering....always thought there was a minimum current where the zener would regulate.
Seeing the high values of the resistors shown, would that be sufficient current..?

8. ### hevans1944Hop - AC8NS

4,645
2,167
Jun 21, 2012
It appears to be there to protect the picaxe A/D input from excessive voltage. My guess is a half-dozen or so series-connected forward-biased diodes would pretty much do the same thing, although the zener should still work too... maybe not as far down on the "knee" as you would like if this were a voltage reference, but its just there for safety and will probably work just fine "starved" for current.

Bluejets likes this.
9. ### BobK

7,682
1,688
Jan 5, 2010
Yes, under normal circumstances, the Zener will not conduct at all. Only if the divided voltage exceeds the 5.1V Zener voltage will it start conducting. The PIC allows inputs pins to go up to 0.3V above the Vdd, so with the PIC powered at 5V this protects the PIC input from excess voltage, clamping it at 5.1V which is safely under the allowed 5.3V.

Bob

Bluejets likes this.
10. ### JPU

281
1
May 19, 2012
Hi BobK and all.

Thank you so much for your time and help with this.

JPU