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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.

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  • \$\begingroup\$ Are you looking for a Schmitt trigger? \$\endgroup\$ – Andrew Morton Mar 9 '15 at 22:04
  • \$\begingroup\$ I suppose I am, now that I know what that's called :). Mind expanding on that as an answer? \$\endgroup\$ – kolosy Mar 9 '15 at 22:09
  • \$\begingroup\$ I think it might be better to wait for replies from people with more knowledge on this than me ;) \$\endgroup\$ – Andrew Morton Mar 9 '15 at 22:11
  • \$\begingroup\$ @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 :) \$\endgroup\$ – bitsmack Mar 9 '15 at 22:13
  • \$\begingroup\$ 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. \$\endgroup\$ – WhatRoughBeast Mar 9 '15 at 22:17
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A simple comparator circuit will do:

schematic

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.

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  • \$\begingroup\$ Awesome. What's the purpose of C1 here? \$\endgroup\$ – kolosy Mar 9 '15 at 22:12
  • \$\begingroup\$ C1 is a decoupling capacitor and isolates the dynamic currents needed by U1 from influencing the rest of the circuit. \$\endgroup\$ – Kuba Ober Mar 9 '15 at 22:14
  • \$\begingroup\$ @KubaOber And, as a decoupling capacitor, should it be placed as close as electrically possible to U1? \$\endgroup\$ – Andrew Morton Mar 9 '15 at 22:16
  • \$\begingroup\$ 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. \$\endgroup\$ – WhatRoughBeast Mar 10 '15 at 1:41
  • \$\begingroup\$ @WhatRoughBeast I'm presuming, perhaps wrongly, that the part will be used according to its datasheet :) \$\endgroup\$ – Kuba Ober Mar 10 '15 at 14:33
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You're looking for a "comparator":

enter image description here

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:

enter image description here

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