Wednesday, January 5, 2011

555 propagation delay

Still using the same 555-relay circuit I did a "prequel" and monitored the propagation delay between S(et)-R(eset) inputs and 555 output state change. Channel 1 of oscilloscope was connected to either pin2 or pin6 of the 555, while channel two was connected to pin3. Ground clips were connected to circuit ground. Trigger mode was of course single sweep, rising or falling edge triggered, and triggering on channel 1. Trigger voltage was adjusted accordingly depending on whether rising or falling edge trigger was selected. Each screenshot below is a different, unique acquisition.

The switch used for S and R were simply short lengths of  AWG#22  solid wire. One end was plugged into the breadboard slot for pin2/pin6 and the other end was momentarily plugged into the ground/VCC slot when the 555 was to be triggered. What amazed me--and as you will see in the shots below--was that there was no contact bounce at all with this most primitive method of switching.

A. Channel 1: 555 pin2, falling edge trigger.
Propagation delay, measured @50% of VCC:  ~200ns

B. Channel 1: 555 pin6, rising edge trigger
Propagation delay, measured @50% of VCC:  ~175ns 

Interestingly, there's very noticeable ringing when pin3 goes from high to low. And it begins the moment pin6 goes high. Quite intriguing. Wonder what's happening internally.

All of the tests above were done with the 2N7000 and relay connected. They were disconnected from the circuit and the results were still the same including the observed pin3 ringing.

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