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This is a re-modelled subset of an actual “Watchdog Timer” circuit that I built recently, using material from Parr’s classic “IC 555 Projects”. Only lamps and LEDs are used for the various status indicators here, but the much bigger real circuit includes parallel amplified sound generator branches for each of them. There are two reasons why those branches are not included here: there is no further room in the workspace, and anyway EveryCircuit does not appear to include any symbols/functionality for sound ICs, amps, speakers or piezo buzzers.
There are three sub-circuits (Heartbeat, Monitoring, and Alarm). They are all already powered on, with the red alarm LED lit (i.e. “Alarm un-set”) and a “heartbeat” visibly active.
The Heartbeat circuit on the left represents a part of any system or equipment capable of sending out a regular “I’m still healthy” pulse. This one happens to run at around 70 beats per minute, just like a healthy human heart at rest. The heartbeat is indicated on the main bright white lamp, assumed to be located in the same room as its “system”, and also on two “amber” lamps, each assumed to be in different locations (e.g. doctor’s or engineer’s offices). The 9v DC power supply to this 555 astable timer sub-circuit is enabled with the lower of the two switches located to the right of the voltage source symbol and master switch. Leave it powered on for now.
The purpose of the Monitoring circuit in the centre is to check that the heartbeat is still pulsing. This is a 555 monostable timer sub-circuit, re-triggerable via the two NPN transistors. It is powered with the upper of the two switches located to the right of the voltage source symbol and master switch. Leave it on for now.
These two sub-circuits are not yet linked up. And first we should test the monitoring system itself. Pressing the leftmost button at the bottom simulates just one heartbeat. The two green “All OK” LEDs next to each of the amber lamps (in the “remote” offices) at once come on solidly for around seven seconds (see later), and then go out. Now press that triggering button again, but this time also then quickly press the alarm reset button to the right of its red LED. The red LED turns off and the alarm circuit’s green LED comes on. All is well … but only until the seven seconds have elapsed. Then all three green LEDs go out again, and the red LED comes back on.
Now that we know the monitoring circuit is working, we can join the heartbeat circuit to the monitoring circuit by flicking on the linking switch located near the centre, to the right of the main heartbeat lamp. The two green monitoring LEDs should come on and STAY ON, showing all is well with the monitored “system”. Now press the alarm reset button (see above), which turns off the red LED and turns on the green LED.
The Alarm circuit, which uses a two NAND gate SR latch, is powered (for its reset function) via the switch located at top right. Leave it on for now. But before we can trust the Alarm system and its indicators (assumed to be in the security guard’s office), we should specifically test it. Press the monitoring reset button (the second button from the left at the bottom). The central green LED should go out and the red LED next to it should come on. But the green heartbeat monitoring LEDs are both still on. That’s good: it’s only a test, no need for anyone to panic. Now we (the guard) can reset the Alarm circuit by pressing the switch to the right of the red LED. The red LED goes out and the central green LED comes back on. “All set”.
Everything is now operating as it should, and in the full circuit there are (different) pieces of gentle music playing alongside each green monitoring LED. There’s no need for anyone to watch the lights continuously!
If and when the main “system” develops a fault and stops sending out a pulse, the monitor circuit waits for seven seconds, to allow the system to recover automatically, if it can. You can (crudely) simulate this by flicking the central linking switch off, and then back on again after three or four seconds. Nothing should have changed: the monitored “system” has recovered. But if you flick that switch off again, and leave it off, then after seven seconds all the green LEDs go out (and all the music stops!), and the alarm is “sounded” ..... with the red LED in this circuit, and also with a loud siren in the full circuit.
Later, once the engineer or doctor has cured the problem, and the green LED status indicators (and music) are on again in their offices (flick the linking switch back on to simulate this), the security guard can clear the Alarm by pressing its reset button, and we are back to “All set” again.
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