# Is there a circuit where output signal is high ONLY when at a specified input voltage?

Is there a circuit where the output is high (or on) when the input voltage is at a specific voltage range? For example, if the input voltage is 4.5 V or lower than the circuit output is low (or off). And if the input voltage is 5.5 V or higher than the circuit output is low (or off). However, if the input voltage is between 4.5 V and 5.5 V then the circuit output is high (or on). I've tried various ways using Zener diodes and transistors but keep running into dead ends. Any help is appreciated.

• google window comparitor. – owg60 Apr 11 '17 at 21:41
• Try a couple of analog open collector output comparators with their outputs tied together. perhaps an lm393 – Trevor_G Apr 11 '17 at 21:43

You are looking for a window comparator. It can be implemented easily with a dedicated window comparator IC (like the TPS3700) or a dual comparator IC (or two single comparators). The two comparators should be open collector (i.e. not push-pull) so that the two outputs don't fight each other. Here's an example from the TLV170x comparator datasheet: Since the two thresholds (you're using 4.5 V and 5.5 V) connect to the high impedance inputs of the comparators you don't have to buffer them. You can use a resistor divider, composed of three resistors in order to set two thresholds, from your supply voltage to ground. The datasheet for the aforementioned TPS3700 shows this: In the event that the comparator supply voltage $V_\text{S}$ is less than your input thresholds you can divide down your the thresholds and your signal to less than $V_\text{S}$. For example, if you have $V_\text{S} = 2\text{ V}$ then you can divide your 4.5 V and 5.5 V thresholds by, say, 4 -- that would give you thresholds of 1.125 V and 1.375 V.

• ADDITION : Good answer but it assumes Vs > Vin. He may need a divider and a voltage reference device if this needs this to be powered from the same Vin voltage. – Trevor_G Apr 11 '17 at 21:54
• @Trevor Good point. I've added that information. – Null Apr 11 '17 at 22:02
• That looks suspiciously like a 555. – Bryan Boettcher Apr 11 '17 at 22:13