# Digital circuit to compare two voltages

I have a task in which I have two sensors, each reading a voltage value. I want a circuit to output 1 if the difference between the two readings is smaller than 10% and 0 otherwise.

I can solve it both analogic or digitally. Since I have more experience dealing with digital circuits than analogic ones, I thought about doing the following: Convert the readings to binary and build a circuit which basically uses the formula | (V1 - V2) | / ((V1 + V2)/2) ) and compares it with 0.1. I think there is an easier way of solving this and wanted some help to find it.

Any help will be greatly appreciated.

• The AD633 analog multiplier datasheet has application notes on how to use it for multiplication and division. It might be worth a look. It might persuade you to go digital! – Transistor May 19 '16 at 23:15
• What do you want to happen if the two voltages approach 0 V? – The Photon May 19 '16 at 23:18
• @ThePhoton If they both approach 0 V, I would want to output 1. – tsuo euoy May 19 '16 at 23:26 simulate this circuit – Schematic created using CircuitLab

Figure 1. Dual comparitor circuit using open-collector outputs to form logical AND function.

How it works:

• R4 and R5 divide V2 by 2 giving 50% of V2 at the junction.
• R1, 2 and 3 set reference points of 55% and 45% of V1.
• The LM324 has open-collector outputs so they can be connected together and pulled high by R6. If either comparitor switches low the output will pull low without damage to the other comparitor.
• CMP1 will switch low if V2/2 > 55% of V1. This is equivalent to V2 being > 110% of V1. OUT = 0.
• CMP2 will switch low if V2/2 < 45% of V1. This is equivalent to V2 beint < 90% of V1. OUT = 0.
• If 90% < $\frac {V2}{V1}$ < 110% then OUT = 1.

I chose the resistor values to make the % calculations obvious. You can change them by scaling them up or down but keeping the ratios in each chain (and then select the nearest standard value). e.g. R4 and 5 = 10k. R1 and R3 = 47k.

• Thank you so much for taking the time to do this and explain it so detailed! – tsuo euoy May 19 '16 at 23:47
• Thanks for accepting my answer but it may discourage other answers to your question. Usual advice is to wait a day or two to see if anyone else responds. It also gives others a chance to point out errors in my answer. You should figure out what happens with this circuit as the voltages approach zero too. I didn't give it much thought. So, if you wish to un-accept my answer (for a while!) then that's fine. – Transistor May 20 '16 at 2:49
• For a real circuit, you should probably add some hysteresis. This will add interesting complexity to exactly defining the trip points in terms of percentage. – WhatRoughBeast May 20 '16 at 4:54