555

• 200mA
• max frequency 500kHz - 2MHz

Pins

1. Ground
2. Trigger
3. Output
4. Reset
5. Control Voltage
6. Threshold
7. Discharge
8. Vcc

When the trigger is below 1/3 Vcc, output goes high, and stays high. When the threshold pin goes above 2/3 Vcc, output goes low.

Output can handle up to 200mA.

The control voltage can set the 2/3 level directly. Often it is not used and connected to ground with 10uF cap.

When the output is low, the discharge has a low impedance connection to ground.

Besides the basic modes (monostable, bistable and astable) it can be used for digital logic. The piece that turned me on to this was using it as a temperature controller. Where the 1/3 level corresponds to "turn on the heater" and the 2/3 level corresponds to "turn off the heater". Any logic of that nature. But that involves some tricky resistor math to convert the voltage being received to the desired voltage.

If a signal is fed into Control Voltage with the circuit in monostable mode, it becomes a class D amp. I'll put this note here, this took me a while to understand; the out put of a PWM encoder, to convert back to analog, needs to go through a resistor and a diode, and there needs to be a cap in parallel with the load.

On the one I tested, there was a small voltage drop at the output.

I need more testing, but it seems like changing the ctrl by too much just doesn't work. I tried setting it above Vcc and nothing really worked.

States

If the Threshold is High, the Output is always Low.

If the Threshold is Low and the Trigger is High, the Output is High.

If both are low, the output keeps it's previous state.

NOTE: I think I need to test this again in ternary.

Questions

If BOTH trigger is low and threshold is high, which take precedent? Based on the bistable configuration, I believe that threshold takes precedent.

I don't completely understand Control, it's intended to be used to set, but can we read a voltage there too? Can the control voltage be higher than V+ (to use the 555 to step down an output)?

PWM

The basic a-stable mode can only do PWM above 50% duty cycle. That can be reduced with a diode across R2, I need to experiment with that.

For a more accurate PWM a 556 can be used with the first half in a-stable mode for the frequency and the second half in mono-stable mode to set the pulse per cycle length. A large part of what I want this for is a MOSFET driver, I'm curious if I could integrate a charge pump into the first stage.