Electronic – Need second set of eyes on 555 50% duty cycle soldered protoboard

555pcb-design

I'm a hobbyist trying to learn the basics of the 555 timer, and am working off of the 50% duty cycle circuit from the National Semiconductor data sheet for the 555 (PDF pg. 10 Fig. 14) and have successfully prototyped the circuit on a breadboard, but am having difficulty identifying why my soldered protoboard version isn't working properly. Here's what I've looked at:

Schematic from the datasheet:

Datasheet Figure 14

Below is how I've laid out the components and soldered them to the protoboard (as part of a larger design which is why explicit V_cc and GND are omitted):

enter image description here

(Bold lines are soldered connections, arrows are wire based connections. Red for V_cc, Black for ground, blue for intermediate connection and gold for primary output.)

  • Pins 1, 5 are connected to ground (I recognize that 5 should go to a capacitor to ground, but have omitted it here as it worked without it on the breadboard and space was limited)
  • Pins 2, 6 are connected together and their junction is placed between R_A = 10 k Ohm and C = 100 uF.
  • Pin 3 is outbound.
  • Pin 4 and 8 are connected together and to R_A with a 5V regulated source.
  • Pin 7 is connected to R_B = 2.2 k\Ohm which is in turn placed between R_A and C.

I've gone through and taken voltage measurements and from ground

  • Pins 2 and 6 are 0.9V
  • Pin 3 is 0V
  • Pins 4, 8 are 5.01V
  • Pin 7 is 0.01V

My first thought was that I damaged something during the soldering process, so I went through and verified that the resistors and capacitor were functioning. The 555 sits in a socket, and each of the socket connections is going through.

So the big question is, what did I screw up in translating from the datasheet to this design? And, in general, what tests can I conduct myself to identify how to solve "why isn't my protoboard working" kind of problems in the future?

Best Answer

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:

enter image description here

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.

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:

  1. Follow the power (ensure power is being supplied, delivered, and propagated as expected)

  2. Control the inputs (use a range of controlled inputs to explore the boundaries of the problem scenario)

  3. Isolate the elements (disconnect key elements to create smaller mini-circuits in the overall network)

Good luck!

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