OR gate with two 555 - del00014
This shows how to OR gate two 555, when one 555
cycles at a low frequency a valve turns on an off,
the second 555 stretches the ON duration of the
pulse with a diode OR gate.
Polarized Capacitors and 555
The OR output uses sample and hold to get the
stable analog data from a sensor after the
actuator has gone OFF, this ensures correct
Frequency Divider 74HCT4040
555 is a fundamental Mixed Signal Circuit as it
can be made into a VCO using Pin-5. If you see old
exar databooks, you can see 555 and PLL and Tone
decoders all applications compiled in one base. I
feel the Venerable Signetics 555 "Architecture"
and Intersil ICL8038 'CMOS' were inspiration
behind early communication chip designs, Moving
from Bakelite Telephones to Compact Push Button
Electronic Phones and more.
Edit the circuit eagle cad file del00014.zip.
Small inductor in the load is to reduce voltage
spikes at switch-on. To Reduce noise use a Gated
This circuit could be modified with a Opto-Isolator
with Zero Crossing with at least 5KV Isolation. The
reason being this circuit can be a safe only if made
into a Hermetically Sealed Module like a Dimmer or
Thermostat. The knob of the selector switch and the
shaft must have 2KV Isolation for product safety.
In Industrial Timers and even Toaster Timers this
"Live" configuration is common. If you are
experimenting with circuits please do it with
Opto-couplers for safety. This applies for
youngsters and students too.
While manufacturing a circuit like this, use a
Overrated Thyristor and Fuse for better safety and
reliability. You can notice a RC Snubber across the
Thyristor to soften Spikes and reduce EMI.
Pin 2 is the trigger input, falling edge of pulse
triggers. Trigger Pulse should be shorter than RC.
Image above from the Original
Signetics 555 Datasheet