On March 9, 2018, I built part of my circuit (from Revision 3) onto a breadboard and began to explore the workings of the 555 oscillator. This oscillator would be responsible for pumping an inductor and boosting 3.3 V to ~425 V.
In this photo, I'm examining the output of the 555 timer, pin 4.
The 555 is wired to operate in bistable
mode, in which it acts like a flip-flop. It
outputs binary pulses, going high when
the trigger pin (pin 2) is activated and going low when the reset pin (pin 4) is activated.
Here is a typical schematic of a 555 timer wired in bistable mode.
Though pin 5 is not used in this schematic, noise on pin 5 can cause issues. It depends on the 555 timer (bipolar vs. CMOS).
“Pin 5. Control Pin: Output Pulse width can be controlled by applying voltage at this Pin, irrespective of RC network. Normally this pin is pulled down with a capacitor (0.01 uF), to avoid unwanted noise interference with the working.”
Quote: https://circuitdigest.com/electronic-circuits/555-timer-bistable-multivibrator-circuit-diagram
Also note that, on my breadboard, pin 7 (discharge) and pin 3 (output) are connected. These two pins are essentially the same, with a minor difference: pin 3 can go low and high, with some time delay between those states. Pin 7 can go low or open. Pin 7 has been enlisted to pull the high pulse low very quickly, which collapses the current flowing through the inductor faster and increases dI/dt.
This is what we expect to see.
“Trigger (pin 2) makes the output high.
Trigger is 'active low', it functions
when < 1/3 Vs.
Reset (pin 4) makes the output low.
Reset is 'active low', it resets when <
0.7V.”
Image and quote: https://electronicsclub.info/555bistable.htm
What I saw. This circuit is clearly sending reset signals, but something weird is going on. Time to check that I have everything wired up correctly. (Spoiler alert: I didn't. Can you spot my mistake?)
Comments