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I'm trying to build the above circuit, which is supposed to flash an LED with a schmitt trigger (with the possibility to vary the frequency with the knob of the 47kOhm potentiometer). I don't have a 12V lamp so I replaced it with a red LED. I added it in series with a 1K resistor. The rest is exactly as on the picture.

When I turn on the power, the LED comes on with one intensity and stays like that. Also no matter where I turn the knob of the potentiometer, the intensity of the LED stays the same.

I've triple, quadruple, quintouple checked my connections and they're correct.

Please help. What am I doing wrong?

Edit: I have a 9v 0.45A on hand, but it doesn't even turn on with the upper configuration.

  • 1
    \$\begingroup\$ actually that circuit is supposed to flash a 0.6W incandescent lamp, \$\endgroup\$ – Jasen Dec 23 '18 at 19:43
  • 2
    \$\begingroup\$ if you have a LED where the lamp is drawn add 470 ohms in series with the led and 220 ohms parallel with that new resistor and the led \$\endgroup\$ – Jasen Dec 23 '18 at 19:47
  • \$\begingroup\$ @Jasen that still doesn't make it a relaxation Osc. which relies on the PTC effects of tungsten and size , power and voltage rating of bulb, Better to use a Schmitt logic Inverter and power FET if necessary with 10M feedback pot and Cap input to Gnd for RC=T \$\endgroup\$ – Sunnyskyguy EE75 Dec 23 '18 at 20:06
  • \$\begingroup\$ I'm not convinced that circuit works all that well even with a lamp. \$\endgroup\$ – Spehro Pefhany Dec 23 '18 at 20:10
  • \$\begingroup\$ You cannot replace an incandescent lamp with a LED and resistor, the two act quite differently. You have seriously modified the current flow. Look for an LED flasher. \$\endgroup\$ – Jack Creasey Dec 23 '18 at 21:19

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