Battery life and continuity indicators using LED's

[mm85]

Battery life and continuity indicators using LED\'s

I am working on a project which uses a 7.2v battery, and I was wondering if someone could give me a schematic or maybe some info on how to make a battery life indicator using 4 leds.

I also have another concern which I'm hoping someone can provide an answer to since I'm not too experienced yet with electronics. I am making a handheld ignition unit to control some pyrotechnic devices, and I would like to have an led on each of the three channels showing continuity status of the ignitors. My problem is I need to pass enough current for the led to light but not so that the ignitor ignites. Would I just figure the resistance of the ignitor and add the proper resistor value in series so that current is at the 20mA the led is rated at? Or would I do this a different way?

 

[ODatsBright]

Re: Battery life and continuity indicators using LED\'s

20mA should be ok through a standard model rocket igniter but personally to minimize the risk I'd use some kind of logic circuit in the µA range to then turn on an LED.

From what I remember the simplist indicator was different colored LED's and a divider network of diodes, but I don't think that is the best circuit for actually indicating battery health.

 

[mm85]

Re: Battery life and continuity indicators using LED\'s

Do you know where to find a schematic of a logic circuit? How could I go about having just a simple battery power indicator, maybe like two green leds, a yellow, and a red?

 

[Flatscan]

Re: Battery life and continuity indicators using L

You'll get better responses to this in Electronics. A mod will probably move this eventually.

I think there may have been a thread on multiple LED battery indicators, but I haven't searched. I started kicking the idea around a little while ago and thought up an obvious solution. I know that it's easy to build and will work, but I don't know how good it is.

I'm being careless with my terminology here because I don't remember specifics. You'll need to figure out low/high Vgs -> switch on/off -> LED on/off and select appropriate transistors.

A battery power indicator would be most likely based on battery voltage. If you have voltage-gated switchs (transistor/FET) and appropriate voltage dividers, you can set it up so that each LED is on above an individual threshold voltage and off below that. If you have enough Vbatt to drive all the LEDs in series, you can wire the switch/LED pair in parallel so that the switch comes on to bypass the LED, turning it off. Otherwise, the LEDs need to be wired parallel to each other, with the switch/LED pair in series.

I assume that the igniter is an open circuit once ignited. You can run a small current through it, for a relatively low voltage drop across the igniter, and feed this to the gate of the switch. If the voltage is low, the LED is on. Once ignited, the voltage across the igniter goes high, and the LED turns off.

 

[MikeLip]

Re: Battery life and continuity indicators using L

Be careful on igniter squib currents. We used them (although probably not what you are using) at Westinghouse to fire our motors - we made torpedos. To check continuity we used a low current meter - as I recall, currents were in the microamps. A regular VOM would set one off. So ignition currents in our case were very low. Just something to keep in mind, and information that should be easily available from the manufacturer if it's not printed on the box somewhere.

 

[UK Owl]

Re: Battery life and continuity indicators using L

Not sure what you are after, but try this as an extremelly low current continuity circuit. Battery charge level indicators are a bit more difficult so I will leave this for a later day:-

Note: where I say + side of the battery, make sure you put an on/off switch first as an extra safety precaution.

connect the + on the battery to one side of the 'ignitor', connect the other side of the ignitor to the gate of an insulated gate N-Type FET. also connect from this point to the - of the battery via a 10 meg resistor. connect the - terminal of the led to the - terminal of the battery. connect the + terminal of the LED to the drain (-) terminal of the FET. connect the source (+) terminal of the FET to the + of the battery via a resistor (note this resistor is the LED series resistor and will probably be in about the 300 ohm range). You may also put a switch across the 10 meg resistor and use this as the 'launch' switch.


If all goes well it should work, but take extreme care and act like the pyrotechnics will light at any time.

Good luck !

 

[mm85]

Re: Battery life and continuity indicators using L

Thanks a lot for that info, I'm going to put it together very soon. And I use a 10 mega-ohm resistor correct?

 

[MikeLip]

Re: Battery life and continuity indicators using L

That's a good idea. IMHO, the gate to source resistor could be anything from 100K on up to the 10meg. The resistor is just to keep the gate from floating high - FETs have a tendency to come on by themselves if there is no path for the gate charge to go to ground. You just want a high value to keep the current sourced by the continuity checker low. If you use 100K and 3 volts worth of battery, you end up with a max of 30 microamps. That should do it /ubbthreads/images/graemlins/smile.gif