555
I'm trying to build a 555 timer with a 50% duty cycle.
I followed the instructions given here (pg 12).
I used a 470 microfarad capacitor, 150 ohm resistor and a kohm resistor. The output voltage is always low.
I've attached an image of my current circuit and would appreciate it if someone could pinpoint what I'm doing wrong.
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) < "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.
There are a bunch of app notes and datasheets available here.
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!
If the duty cycle itself does not matter and the requirement is for pulses of, say, 0.5ms, then you could send the output of your circuit to a monostable 555 circuit. Dual-555 chips are available: the 556.
Best Answer
If you need an accurate 50% duty cycle, a simple 555 circuit is not the way to go. A better method is to have an oscillator run at double frequency, where duty cycle is unimportant. Then feed that signal in a divide-by-two circuit and you'll have a perfect 50% duty cycle.
A simple divider can be made using a JK-flip flop where the J and K are high, like in this circuit:
simulate this circuit – Schematic created using CircuitLab
There are countless ways to create a 2-divider, all with their own advantages and disadvantages. A simple Google image search will give you pages full of them.