The circuit uses a small single pole relay - such as the 12v Omron G2E. The coils of these low voltage DC relays behave just like resistors. I've used a 270 ohm resistor (Ry) to simulate my relay coil. The Differential Voltage Probe measures the voltage across the relay coil. And the state of the relay is determined by the level of that voltage.
The graph below represents two separate activations. The first occurs at 30 seconds - when Sw1 closes briefly. And the second occurs at 150 seconds - when Sw1 closes and remains closed. In each case - the voltage across the coil rises sharply to 12v. This causes the relay to energize - and the siren to sound.
In the case of the first activation - where Sw1 is re-opened - the siren cut-off time is short. The noise will stop after about ten seconds. Where Sw1 remains closed - the cut-off time is longer - about sixty seconds. The top line represents the trigger pulses. I've just shown two activations. But the alarm will go on activating every time Sw1 closes.
The brief On/Off pulse is provided by V3. Thirty seconds into the simulation - it turns Sw1 on. The switch connects C1 to ground - charges C2 - turns Q1 & Q2 on - and activates the alarm. Five seconds later - V3 re-opens Sw1. And C2 begins to discharge slowly - through R2 & R3. About ten seconds later - the relay drops out - and the noise stops.
The Trigger Pulse Settings (V3)
The longer - second pulse - is provided by V4. One-hundred and fifty seconds into the simulation - it turns Sw1 on. As before - the switch connects C1 to ground - charges C2 - turns Q1 & Q2 on - and activates the alarm. But this time the "width" setting is two-hundred seconds. So Sw1 remains switched on - beyond the end of the simulation.
The Trigger Pulse Settings (V4)
As before - the siren will stop when C2 discharges through R2 & R3. However - this time - C2 discharges much more slowly. It must wait while C1 completes its charge. This takes about a minute. Then the relay will drop out - and the noise will stop.
The analysis is fairly straight forward. The "Mode" should be set to "Transient". And the "Stop Time" should be long enough to allow the alarm to complete the two activations. I chose 300 seconds.
SIMetrix uses your choice of "Stop Time" to calculate some of its "default" settings. If your choice of figure is high - some of the default settings will be too high. And this will prevent the simulation from running successfully.
You could solve the problem by changing some of the individual default settings - such as the minimum time step - or the integration method. But for present purposes - I found that it was sufficient to increase the value of the "relative tolerance". All the other settings can remain at their default levels. You'll find the relative tolerance - under the "options" tab.
Why Are There Diodes In Series With V3 & V4?
I wanted to switch Sw1 twice - first with the brief "On/Off" pulse - then later with the longer "On Only" pulse. So I needed two separate waveforms - from two separate waveform generators. The diodes create one-way paths. They allow both control pulses to reach Sw1 - but they prevent the individual waveform generators from shorting together.
When the V3 pulse is high - V4 is low. So the high V3 pulse would simply disappear into the low V4 generator. The converse is also true. When the V4 pulse is high - V3 is low. So the high V4 pulse would simply disappear into the low V3 generator. The diodes stop this from happening. While the V3 pulse can reach Sw1 - it cannot reach V4. And while the V4 pulse can also reach Sw1 - it cannot reach V3.
The Thyristor (Ty1)
The model library that comes with SIMetrix 5.40 doesn't include an SCR. It has to be created as a separate sub-circuit. I created my SCR sub-circuit - using a Complementary Latch
. I'm not suggesting that it's a very satisfactory general purpose SCR model. But it's perfectly adequate for present purposes. If you want to examine the SCR sub-circuit more closely - you'll find it in the download file.
To run the simulation successfully - you'll need to install my SCR model. This is very easy to do. Open SIMetrix - and simply drag and drop the "MyScr.lb" file into the small "SIMetrix Command Shell" window. Then when you're asked - confirm that you want to install the new model. Don't Worry! You're not overwriting anything. All the existing models are still there.