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:


(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?

  • 1
    \$\begingroup\$ You've discovered the "mind-reading" circuitry attached to pin 4! It is able to see what you want and gives it to you. You can't imagine how much design work went into that feature! More seriously, pin 4 should be connected to Vcc and should not be left open to touching by your hand. The first thing that comes to mind is that when using a resistor in the megohm range, parasitics may be a problem. (I've not verified the frequency you expect.) \$\endgroup\$
    – jonk
    Jul 11 '19 at 23:01
  • \$\begingroup\$ Sound like a bad connection or something of that ilk. \$\endgroup\$ Jul 11 '19 at 23:06

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.

  • \$\begingroup\$ i tried these new values and had the exact same result actually. It still only works when I disconnect pin 4 from V+ and touch the jumper wire. Not sure what else it could be? \$\endgroup\$
    – wirl
    Jul 15 '19 at 21:56
  • \$\begingroup\$ Is this hooked up on a breadboard? what power supply are you using? Do you have a capacitor across the supply on the device pins? \$\endgroup\$ Jul 15 '19 at 22:00
  • \$\begingroup\$ yep, i have it hooked up on a breadboard. i'm using a 5v power supply. im getting power from a voltage regulator in a circuit on a cassette deck. my wiring is exactly as it shows in the diagram above, so no cap across device pins \$\endgroup\$
    – wirl
    Jul 15 '19 at 22:05
  • \$\begingroup\$ (there's also an 8v and 15v voltage regulator on this circuit which i could take power from if you think having higher voltage would help) \$\endgroup\$
    – wirl
    Jul 15 '19 at 22:06
  • \$\begingroup\$ It's certainly worth trying 8 V at least and find out if you have a power supply problem. If you are not sure of the quality of the supplies it might be worth using a simple 9V battery to test the breadboard. \$\endgroup\$ Jul 15 '19 at 22:14

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