For the first two thirds of my professional life, I was a consumer electronics repair technician -- not many opportunities to design circuits. For the final third of my professional career, I was a field application engineer for a test equipment company. Part of my job was to develop the hardware and software (printed manuals, not computer software) for training courses. This allowed limited opportunities for circuit design. As an amateur radio operator for 40-plus years, this provided some circuit design experience. Therefore, when presented with an opportunity to design a small control module, I felt qualified to accept the assignment.
The control module was a simple timer circuit that would activate a relay that would switch a couple additional modules on a five minute on, 55 minute off sequence as long as power was applied to the circuit.
Typically, an MCU (Microprocessor Control Unit or uP) is used when the timing needs to be precise. Precision was not critically important in this application. Therefore, I chose to use an inexpensive timer circuit using the NE555. One 555 IC, 1 bypass capacitor, 1 electrolytic capacitor, and two resistors -- very simple and proven circuit.
The on/off times and duration between the pulses is determined by the values of the electrolytic capacitor and the two resistors. After calculating the values needed, I simulated the circuit to verify proper operation.
Next step, breadboard the circuit. I had several small solderless breadboards that would work for the timer circuit, but I wanted to add the modules that the timer would control to the breadboard, so I ordered a couple larger solderless breadboards.
I wired the timer circuit on the breadboard, connected my scope to the output of the 555 IC, and applied power. As expected, the output went high for the expected time, plus a little extra time since the electrolytic had to charge from a fully discharged state.
Once the output went low, I disconnected the scope and attached a buzzer to the output, so that I could be alerted when the circuit went high again.
I got busy in my lab working on other projects and almost forgot about the timer circuit. I looked at the wall clock -- almost three hours had passed and the buzzer had not activated. Time to check the circuit!
I carefully checked my wiring and verified that everything was connected correctly. All the components were marked with the correct values. Scratching my head, I reapplied power and once again, as expected, the buzzer sounded. I put up with the noise for about six or seven minutes and then it went silent. Once again, I got busy with other projects but kept my eye on the clock. An hour passed and no buzzer noise. An hour and half passed and still no buzzer. Something was definitely wrong. Since it was late in the day, I disconnected power and closed up the lab for the night.
The next morning, before powering up the circuit, I exchanged the 555 IC with another one. Using an ohmmeter and capacitor checker, I checked the components. They all tested within tolerance. I applied power and was rewarded with the annoying buzzer. As before, within six or seven minutes, it went silent.