Electronic – 555 timer only works when I disconnect pin 4 from V+ and touch the jumper

555

I set up a pulse circuit on a breadboard with a 555 timer. Input voltage is 5 V. My values are roughly:

  • C = 100nf
  • R1 = 1K
  • R2 = 1M

I'm using this schematic:

Schematic

(Source: https://www.allaboutcircuits.com/tools/555-timer-astable-circuit/)

This should produce 3 – 4 Hz at the output. When I power up the timer I get a frequency generated at about 150 Hz. Strangely when I disconnect the jumper from V+ (leaving it connected to pin 4), and touch the exposed end of the wire, the circuit works perfectly – outputting 3 – 4 Hz.

This is very weird to me. I tried some different resistor values and capacitors and it's the same result every time.

Any ideas?

Best Answer

1M Ohm is way too large for R2.
You should not have R2 above 100k Ohm with a 5V supply. The leakage currents are killing you.

Try R2=100k and C=1uf

If you have restrictions on the size of components, then you could try the CMOS version of the 555, the LMC555. The leakage currents are in the pA range for this device so you can use large value resistors in the MOhm range with a 5V supply.

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