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.
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:
Q2: 59.5Hz (duty cycle 17%)
I don't understand what can cause a wrong output at Q2?
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:
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?