# Replacing a tactile switch with an ultrasonic ranger

I'd like to have a switch activated when something gets near it. Today it's just a pushbutton, that's active as long as it's depressed.

I'm going to use an analog ultrasonic ranger, using its analog output. What I'd like to do is have an analog circuit activate a mosfet when a certain range threshold is hit. My question is - how do I make this "binary?" How do I have Vgs applied to the mosfet when the analog voltage on the ranger crosses a specific value? I thought about using a zener diode with the breakdown voltage at the distance that I need (the threshold I'm looking for is about 10" away from the device, which translates to ~100mV on the analog pin). Given the low voltage the sensor will output at that distance, I assume a zener won't work.

I'd like to not involve an MCU if possible, because I'd like this to be a simple drop-in.

• Are you looking for a Schmitt trigger? – Andrew Morton Mar 9 '15 at 22:04
• I suppose I am, now that I know what that's called :). Mind expanding on that as an answer? – kolosy Mar 9 '15 at 22:09
• I think it might be better to wait for replies from people with more knowledge on this than me ;) – Andrew Morton Mar 9 '15 at 22:11
• @AndrewMorton is right. I posted my answer before I saw his comment. Basically, you'll use a comparator (see my answer) to build a Schmitt trigger :) – bitsmack Mar 9 '15 at 22:13
• Also, please note that the data sheet indicates you will not be able to tell the difference between 1 inch and 6 inches. Any range of 6 inches or less will be reported as 6 inches. This may or may not be a problem, but you need to be aware of the limitation. – WhatRoughBeast Mar 9 '15 at 22:17

A simple comparator circuit will do:

simulate this circuit – Schematic created using CircuitLab

U1 can be a slow, rail-to-rail CMOS part that costs under \$1 - either a general purpose op-amp, or a dedicated comparator. Ensure that there's no diode between (+) and (-) inputs. Microchip has plenty of them, for example.

Since we assume push-pull outputs like on every op-amp, there's no need for any pull-up/pull-down resistors on the output of U1. If you use an op-amp, that'll be the case. With comparators - it varies.

The ratio of R3 to R4 sets the amount of hysteresis - here it's at 1%. The turn-on voltage is set by the ratio of R1 to R2, and given by VCC * R1/(R1+R2) * (1 - R3/R4). R5 isolates the gate capacitance from the amplifier's output, preventing the amplifier from oscillating.

• Awesome. What's the purpose of C1 here? – kolosy Mar 9 '15 at 22:12
• C1 is a decoupling capacitor and isolates the dynamic currents needed by U1 from influencing the rest of the circuit. – Kuba Ober Mar 9 '15 at 22:14
• @KubaOber And, as a decoupling capacitor, should it be placed as close as electrically possible to U1? – Andrew Morton Mar 9 '15 at 22:16
• Actually, a general-purpose op amp won't work here. The sensor output at 10" will be 100 mV or less, so you need rail-to-rail input capability. – WhatRoughBeast Mar 10 '15 at 1:41
• @WhatRoughBeast I'm presuming, perhaps wrongly, that the part will be used according to its datasheet :) – Kuba Ober Mar 10 '15 at 14:33

You're looking for a "comparator":

You apply a voltage to the Vin- pin which sets the threshold. This voltage is usually derived from a resistor divider. Then, your analog signal is connected to the Vin+ pin.

Choose a comparator with push-pull outputs, instead of open-drain. This way you won't need pullup resistor on the output.

There are also methods of setting hysteresis, if you need to. Here's a good App Note from Maxim.

This is the general layout: