Right now I am using a 555 circuit to generate a 1kHz square oscillation (not displayed below). Generally the reset is pulled down, but through a button I drive a transistor to output the square oscillation when the button is pressed.

I now want to achieve two things:

  1. Instead of activating the signal by setting the reset to Vdd, I want to deactivate the signal by pulling the reset down to ground, when pressing the button. I tried several things but none worked.

  2. I noticed that when I touch the base of the transistor the 555 outputs something ( a strangely distorted 50Hz signal). So it seems to be possible to drive the transistor by a touch plate (plain metal (cu) plate) for example. How can I achieve that the transistor is driven by a Piezoelement or a touch plate?

Current circuit.

  • 1
    \$\begingroup\$ Did you try simply swapping the transistor and the resistor? You'll also need a base resistor in that configuration to limit the current. \$\endgroup\$ – Dave Tweed Apr 17 '18 at 18:54
  • \$\begingroup\$ Yes, I tried that but my transistor caught fire ;). Is a 10k resistor after the switch enough? \$\endgroup\$ – Pearson Apr 17 '18 at 19:02
  • \$\begingroup\$ For 1, get rid of the transistor, use r1 as a pull up and the switch to ground the reset pin. \$\endgroup\$ – Colin Apr 17 '18 at 19:03

You need a limiting resistor for the transistor, otherwise it will source a large amount of current and burn out. This is just an example circuit below, size the current limiter to the value appropriate to your circuit

You are seeing 50Hz because you have not isolated your test setup from AC mains


simulate this circuit – Schematic created using CircuitLab

| improve this answer | |
  • \$\begingroup\$ Thx! Do you know how I could drive the transistor with a piezo element? \$\endgroup\$ – Pearson Apr 18 '18 at 10:30

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