LED "Armed" circuit

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Well-Known Member
Jun 18, 2003
Reaction score
Note: I am really not sure if this is the right forum to put this, Mods PLEASE move it if this is not the right place

In my build thread for my HPR Cluster rocket, there has been some discussion about arming a timer board. My question is this; Can you install a LED to show when the circuit is armed? What additional components (ICs, resistors or Caps) be needed? Would these additions put too much of a power drain on the 9V battery?

You should have no problem installing a LED in your timer circuit. If you use a Nicad for your timer, which is what is recommended for firing real ignitors, then you will have more than enough spare energy to light even the highest powered LED's. Nicads are pretty stout and worth the investment.

As far as LED's go, I would recommend anything that Radio shack sells. Most LED's run on 1.2-3 volts, so using a resistor will be necessary to drop the voltage down from your battery source.

A LED that is rated at 20ma and 2volts has a resistance of 100 ohms at 2volts. the remaining 7 volts from your 9V battery needs to drop across a resistor that is in SERIES. Probably a 470 ohm 1/4-1/2 watt resistor would do fine and you can go as high as 1k ohm and it will function well. Also note that some LED's, while more expensive, can be bought with built in resistors...just make sure they are rated at your batteries voltage.

Hope this helps,

As a suggestion,, since you are considering this as a saftey indicator, consider two alternatives:

1) A high visibility emitter like the THC3 line from someone like LSDIODES


2) An audible indicator like a small piezo buzzer.

What I am getting at here is that if you want a saftey indicator, make sure bright ambient light does not render it unobserveable.

I strongly recommend this method:

Get a BLINKING LED, and wire it up in series with a Piezo Beeper.

That is because an LED alone is not that noticeable, it is a passive thing you have to think to look at. And it may be hard to see in daylight (if you do not use a Piezo beeper in series, then at least gear a CLEAR LED as those are easier to see as being on or off in daylight than the diffused colored LED’s are).

I use a blinking LED with a Piezo beeper for a number of warning purposes. Mostly as a continuity indicator. Also I use it with an electric powered sailplane, to remind me the power is on so I hopefully won’t turn the throttle on before takeoff or forget to turn the plane off after landing (flew R/C for so long with gliders, I’m not used to the potential for an electric engine coming to life by accident. So anything I could do to try to minimize that, I did).

In fact, I do not even bother to mount the blinking LED where it can be seen, as it is superfluous. There’s no need to look at the passive LED, as the beeper is making itself heard in an active way (if the beeper is not beeping, the LED is not on either). The purpose of the blinking LED is to be the simplest lightest way to make the Piezo beeper make a “beep.....beep.....beep....” sound. The on-off beep repeat cycles are more noticeable than a beeper that is just on all the time (especially at a distance when the beeping can tend to start to be hidden by background noise).

Radio Shack has a 1/2” diameter Peizo Beeper, for around $3. That’s what I use. Works with a wide rang of voltages. I don’t even bother with a dropping resistor for the blinking LED, because when used in series with the Piezo Beeper that is enough of a drop to keep the LED from being damaged. Now if a person was using this for checking continuity on very very sensitive ignitors, then there might be a reason to add a dropping resistor to make the current very very low.

- George Gassaway