Need second set of eyes on 555 50% duty cycle soldered protoboard
The "Control Voltage"

There are probably several problems here... but I'd start by not grounding pin 5.

This is a simplified schematic of what's inside your 555 timer:



The flip-flop is RESET (nominal output low) when "Threshold" (pin 6) > "Control Voltage" (pin 5)

The flip-flop is SET (nominal output high) when "Trigger" (pin 2) &lt; "Control Voltage" (pin 5) divided by 2

By shorting pin 5 to ground, you have forced "Control Voltage" to equal 0 volts.

In your case, you have prevented the timer from ever being able to SET its output by guaranteeing that "Trigger" can't ever be lower than "Control Voltage"/2 -- no possible positive voltage being lower than 0 ("ground").

Float (disconnect) pin 5 to fix. When you do not connect anything to pin 5, the "Control Voltage" defaults to 2/3 of Vcc.

Further Reading

There are a bunch of app notes and datasheets available here.

General Electrical Network Debugging

As to your general question about debugging. I tend to advise students and clients to proceed in this general order:


Follow the power (ensure power is being supplied, delivered, and propagated as expected)
Control the inputs (use a range of controlled inputs to explore the boundaries of the problem scenario)
Isolate the elements (disconnect key elements to create smaller mini-circuits in the overall network)


Good luck!