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 reading.

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.

Trigger Mode

Pin 2 is the trigger input, falling edge of pulse triggers. Trigger Pulse should be shorter than RC. This is used in Monostable 555 to get a pulse at Pin 3. You could also make a key debouncing circuit. When the Trigger input is held low the output Pin 3 will be High and when Trigger goes High output goes low.

Photographic Timer

Small inductor in the load is to reduce voltage spikes at switch-on. To Reduce noise use a Gated Zero-Crossing Switch.

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.

Images above are from the Original Signetics 555 Datasheet

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