Relay holding voltage?

[Brimstone]

Afternoon all, just had a chat with the kind people at picaxe and they have advised to ask you intelligent people on the forum for advise.

I have built a working replica of 'Orac' the super computer from Blakes 7, which is switched on via a 'key' (a reed switch and a magnet in a box) when the box is put on top of the case the reed switch is activated and it comes to life.
Sensor 16 led chaser and 2 sets of 8 red leds flash and the main 'Brain' pulsates, and makes a operating noise, also when you move your hand over a certain area it plays a random phrase from the show as if it is talking to you....

This is all working brilliantly and no problems, put the 'key' on and it works, take the key off and it stops.

One thing that would make it spot on, is when the key is removed it makes a sort of dying sound.

The easiest way I can think of doing this is to not have the 'key' as the main on/off switch, but as a sensor to detect when it is activated.

When the key is not there the voltage to the reed switch is 0v so can loop until it detects a voltage and then flick a relay (to activate the whole lot) then keep looping round working and sensing the voltage, if it is 0v then the key has been removed and it can activate the 'dying' sound and then reset and wait for the key to be put back on.

Now to the question.
The whole lot is running from the standard 3xAAA batteries and can power the whole lot for a few days on one set of batteries.

On the shop site they have TR99-6VDC-SB-CD Miniature DPDT 5A Relay.... But the technical team do not know what the holding voltage of the relay is?
I googled the data sheet and it says voltage 6v and pull in voltage is 75%, which makes it 4.5v , but it does not give a holding voltage.
The chips work down to about 2.5v and do not want the whole thing to switch off too soon.

Has anybody used these relays? or any advice on something that would work better ... Ideally they would hold above 2v and it would all work fine...

Thanks in advance as doing a chip order and can put some of these on if they will do the job....

 

[premelec]

There's no way you can get the relay hold V without specific spec on the unit or simply measuring it... 6 volt relay would not normally hold at 2v - without knowing more about the circuits you control ir's hard to advise... perhaps an optocoupler [LED to transistor or LDR] would work for you down to a lower voltage. Possibly a 3 volt latching relay would work out and save you drain current etc... More detail on circuit please - Using logic level MOSFETs to switch could be a good way to go as you can get them with VGsth down to 1 volt or so...

 

[BESQUEUT]

Maybe use a 3V reed relay ?

This one is less expensive...
turn Off Voltage 0,45 V...

 

[Hemi345]

If you're powering everything from 3x AAA batteries and run time is a few days, I would think some simple transistors would be less hassle.

Here's a schematic from a PIR sensor project I made a few years ago. Replace PIR with your key switch. When key is turned on and PICAXE powers up, pin C.5 is set high. pin C.3 is monitored and when program detects it goes low (key is turned off), a routine is run to play your dying sound and then power the whole lot off by putting C.5 low.

(BTW, C.3 should probably should have a 10K pulldown resistor on it)

 

[Brimstone]

Morning all, thanks for the replies, the circuits are quite basic, but needed to put them on 2 large circuit boards as it needs to look like the original one.
It has a 28x chip to produce the 16 led chaser, and 2 08x chips one to do the 4 led alternate flashing and the other to make the brain light at random.
The sound is on a basic sound module that just loops when power is there.

With your helpful replies I may be over thinking this, with looking at the PIR idea, would it be better to just use my 'key' as a sensor, and then just have each chip detect if the voltage is over 0V and start working, then the PICAXE-14 Audio Project Kit that does the sound could detect no voltage and play the 'dying' sound.....

 

[hippy]

would it be better to just use my 'key' as a sensor, and then just have each chip detect if the voltage is over 0V and start working

That would seem sensible, particularly as you can't turn power off for a digital system and then expect it to do anything much after that.

Keeping the power always on and simply using the reed relay as a pure switch to set its operating mode - active, dying, inactive - would make sense. In inactive mode the PICAXE can drop its current consumption to a minimum to prolong battery life.

           -.- +V
           .|.
           | |
           |_| 10K
            |
            }----> PICAXE input
          .-|-.
          | | |
()-----   |\  | Reed relay
    |_|   | | | 
          `-|-'
           _|_ 0V

 

[Brimstone]

Brilliant, that was exactly as i was thinking.
It was just that I make replicas as close to the original as I can, so each board looks the same and does the same job but using modern technology.
There is a switch to turn off each part (as the noise may get on your nerves after a few hours, or in a convention hall may not need it on) and the lights you can switch off each set, and the battery box I have has an on/off switch on it.
(The prototype I made used one picaxe chip as could get it to do everything, but then would have had to add 'fake' components to the boards to make them look right). can just have the 3 boards check the switch, and it sets them all off, switch (key) is removed and all are reset ready to go again after playing the dying sound ...super......ordered the bits and thank you all again...