I have a device X (talking toy), which has a jack of two wires for an external button. That is, I can short the two wires and device X accepts it as a press of a button.

Now, I want to use device X for it's function, but replace mechanical button with a capacitive sensor. I have options, but lets assume it's a cheap TTP223 module, with GND, VCC and IO pins, operating at 2 to 5.5 V. When I checked with 3.3 V, it gave about 2.64 V logical "1" on IO pin when touched. At 5 V, gives 3.8 V for "1".

The device X's jack gives about 5 V on it's button jack and even 800 Ohm resistor seems to trigger it. About 210 uA current flows through the button when shorted.

As I see it, the sensor will have it's own power supply (voltage is part of the question).

The main question is, what is the best way to convert logical "1" into "press of the button"? Preferably, without the need for multiple voltage levels. What could be good "relay" for the task?


1 Answer 1



simulate this circuit – Schematic created using CircuitLab

Figure 1. An NPN transistor will invert the logic signal from the touch sensor to the toy.

With 210 µA on a short it implies a pull-up of about 12 kΩ assuming a 3 V supply. An NPN transistor will be able to pull that low.

  • \$\begingroup\$ What voltage at V+ and Vcc? (or what is V+?) Is INPUT toy's input? \$\endgroup\$
    – Roman Susi
    Oct 14, 2017 at 9:57
  • \$\begingroup\$ V+ is whatever the battery voltage is in the toy. Vcc is your touch sensor's battery. INPUT is the input to the micro-controller inside the toy. "tip" and "GND" are the jack plug contacts. \$\endgroup\$
    – Transistor
    Oct 14, 2017 at 10:35
  • \$\begingroup\$ Correct. It pulls the micro-input high rather than leave it floating. You just have to pull it low with a switch or transistor. \$\endgroup\$
    – Transistor
    Oct 14, 2017 at 10:53
  • \$\begingroup\$ Thank you very much, worked perfectly! Even with BC547 - the first NPN I prorotyped it with. \$\endgroup\$
    – Roman Susi
    Oct 14, 2017 at 11:41
  • \$\begingroup\$ That was quick. Do you understand how it works? \$\endgroup\$
    – Transistor
    Oct 14, 2017 at 11:42

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