I am trying to implement a circuit that will sense a change in capacitance and reflect it in a change of flashing frequency for an LED.

I used NE555P timer and a 74LS393 counter. The circuit connection is as shown below:

enter image description here

RA and RB are in the Mohm range, C2 is 100nF, Csence is in the range of 10-100pF. The output of the timer is connected to to the counter for frequency division (to make it suitable for the LED). OUT of the 555 timer has frequency of 242.4Hz.

I noticed a malfunction in the counter and I couldn't figure out the reason. I probed the outputs of the counter using oscilloscope:

Out: 242Hz

Q0: 118.4Hz


Q2: 59.5Hz (duty cycle 17%)


I don't understand what can cause a wrong output at Q2?

Sceenshots are below: enter image description here

Additionally, when cascading the two 4-bit counters in the IC, the second counter output is as follow:



Q3: Faulty signal

Q3 is shown below:

enter image description here

EDIT: I found that once I feed the two chips from different channels of the DC source (Different VDD and GND channels, though VDD is 5 V for both), the issue gets resolved.

However, I don't understand the theory behind what happened?

  • 2
    \$\begingroup\$ Are the supplies properly decoupled? I can't remember seeing this unclear tops/spikes with any 74 series counter, but its been a while... \$\endgroup\$ – PlasmaHH Feb 16 '17 at 12:28
  • \$\begingroup\$ @PlasmaHH I didn't use any decoupling techniques for the supplies. I just connected all Vcc to a 5V power supply while all GND to the ground of the supply. Will this create a problem? \$\endgroup\$ – HaneenSu Feb 16 '17 at 12:31
  • 1
    \$\begingroup\$ @HaneenSu: the LS series is for tens of MHz signals, they are rather fast and gulp up a lot of current. As a rule of thumb, alsways decouple unless you have the confidence to not need it. \$\endgroup\$ – PlasmaHH Feb 16 '17 at 12:34
  • \$\begingroup\$ @PlasmaHH I tend to agree, about 20 ms coupled with the "hairs" on the signal. (Wonder how the traces look in min-max mode?) \$\endgroup\$ – skvery Feb 16 '17 at 15:50
  • \$\begingroup\$ @plasmaHH I tried to feed the counter from a crystal oscillator and it works perfectly fine \$\endgroup\$ – HaneenSu Feb 19 '17 at 11:24

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