Which Diodes for building a 24vdc alarm annuciation panel

[bigcanuknaz]

Hi.

We have a greenhouse, and I am rebuilding our alarm annunciation panel. This is the panel that allows us to bypass alarms that feed into our alarm panel, and indicates (currently with a flashing led light) that an alarm has been bypassed, so we don't go home with bypassed alarms. Currently, if an alarm is bypassed, we must either:
1. Fix the alarm condition, and re-enable the alarm (flasher stops), or
2. If we can live with the alarm condition (say because we are not growing in that zone), then we can bypass the flasher for that zone.

Currently, we have a single flasher (for "bypassed alarm warning"), and mechanical indication on each bypass pushbutton (that the alarm is bypassed). These pushbuttons are cheap and fragile, and their deterioration over time is the main reason I am rebuilding the panel.

I want to use a lighted 2 position lever switch from automation direct. This is cheap, robust, and easy to wire.
https://www.automationdirect.com/ad...luminated_-a-_Non-Illuminated/LED/GCX1253-24L
https://www.automationdirect.com/ad...luminated_-a-_Non-Illuminated/LED/GCX1253-24L

When the switch is on (alarm bypassed) the light will go on, indicating it is bypassed. Great!!

Now it struck me, that since I already have a light, I do not need a separate flasher. I can make each of the individual lights flash when they are bypassed. (even better because now I know which zone needs attention) But I still want a steady light, when bypassed, but when we can live with the alarm condition.

Without using a relay for each zone, and to be able to reuse the cheap terminal strip knife switches as "no flash for this zone" switches, I want to:

Wire the main 24vdc power for *all* the switches through a 24vdc flasher (existing). That way when a switch is bypassed, the light will flash. So far so good.

When we want to "live with an alarm condition" we will close the knife switch *for an individual zone* and feed constant 24vdc to the same terminal on the lighted switch to keep the light on steady (meaning, bypassed but living with it) The problem is, then the continuous 24vdc would back-feed to all the other switches as well, so I need a diode to stop this.

What diode do I need to use?
24vdc, and just milliamps for the LED light on the switch.

Sorry for such a simple question. (took horticulture at uni, not electronics...)

Thanks
naz

 

[tronitech]

The 1N4001 will do the job. (or any of the 1N400X series).

 

[KeepItSimpleStupid]

Idec makes some nice switches, but they are about $15.00 USD each, push button, labelable and LED lampable. They do hold up. In a greenhouse, you really might want an IP rating which basically says what kind of penetration is allowed.

I built a simple alarm annunciator back in the 80's which was 3PDT relay based, One pole was the alarm loop, one pole was the latch loop and the other was the lamp loop.

Simple until it came to hooking up the fire alarm contact. That contact we had the need to bypass about 2-4x a year when fire alarm testing was taking place. I made a semi monitored contact, where all 3 contacts of an SPDT relay from the fire panel were brought to the shutdown panel.

I had an alarm light and a loop light. If the fire alarm was disconnected (inside the panel) the loop light was off. It also helped that when the fire alarm was in alarm, the loop light would be out. So, in that case the panel could have shut down because of a fire alarm and the panel would be in alarm.

We had to wait until the fire alarm was re-set to enable the panel.

So, the secondary LED indicator showed OFF when bypassed and off when the that point was in alarm. The Point LED indicated where the alarm was from and was latched.

The secondary LED would also be off if the cable was severed. One wire of 2 could be cut and we'd know about it. It wasn't 100% failsafe.

Fire panels have a resistor across the contact so a lower current flows when the alarm contact is open. The alarm point just shorts the resistor. This is called a monitored contact.

Our safety system could also trigger the fire alarm (separate detectors) as well as being shut down from it.

 

[bigcanuknaz]

Thanks tronitech.

Thanks Keep. Yes, I know a bit about monitoring circuits with resistors. Low=alarm High=short(alarm) Correct resistance=no alarm. Our panel has this capability. I agree. For fire alarms, it is a good idea to be more robust. Luckily, this panel is just backup alarms for our greenhouse computer for temperatures, pressures, etc.

Thanks,
naz