How to wire a flashlight?

[MrForbes]

We put a spotlight on our robot...a cheap $14 unit from Walmart. We only used the LED and reflector from the spotlight. The custom circuit consists of a LM7805 voltage regulator and 3.6 Ohm 5 Watt series resistor, and it also uses a Spike relay. The relay is controlled by a relay output on the roborio.

http://www.simple-electronics.com/20...regulator.html

in case you don't know how to figure the resistance and wire up this type of custom circuit.

[JR0405]

You would need to use a relay wired to the Roborio and power the flashlight through the relay from the voltage regulator module

[SuperBK]

Also, you could use a resistor sized with 12 volts in mind and avoid the 7805. And like Mr. Forges says, control the 12V with a spike. Depending on how much the led draws, some teams have run them off a port of the pneumatics control module, though personally, I would prefer to use a spike.

[rich2202]

Depending upon the amps, you might be able to power it off the 5v supply of the VRM. You would still have to have that power go through a relay.

[Ari423]

Originally Posted by rich2202

Depending upon the amps, you might be able to power it off the 5v supply of the VRM. You would still have to have that power go through a relay.

I thought that custom circuits powered through a Spike would need to be powered off the 12V PDP output. I guess you could consider the Spike as part of the custom circuit and power it all off the VRM, but I don't know how much the inspectors or Spike will appreciate that.

[rich2202]

Originally Posted by Ari423

I thought that custom circuits powered through a Spike would need to be powered off the 12V PDP output.

There is no requirement that a Custom Circuit be powered off a 12V PDP output. The rules say a Custom Circuit can have up to a 40A breaker. I could not find a rule that says a Custom Circuit cannot be connected to the VRM.

However, looking at the Spike, it needs at least 9.5 volts, so it will not work switching 5 volts.

So, the custom circuit would need its own custom switch.

[Peyton Yeung]

Both when I was a senior in high school and this year (junior in college) we used a spike relay and voltage converter to turn on the flashlight through the PDP.

We used this flashlight. It's a 3V flashlight so we used this voltage converter. The flashlight was cut in half through the body and wires were added to the + terminal and the body (- side). This was connected to the voltage converter which was connected to the spike relay.

[TimTheGreat]

Originally Posted by Peyton Yeung

Both when I was a senior in high school and this year (junior in college) we used a spike relay and voltage converter to turn on the flashlight through the PDP.

We used this flashlight. It's a 3V flashlight so we used this voltage converter. The flashlight was cut in half through the body and wires were added to the + terminal and the body (- side). This was connected to the voltage converter which was connected to the spike relay.

...Are the voltage converters from the years of the cRIO allowed? We still have ours.

[rich2202]

Originally Posted by TimTheGreat

...Are the voltage converters from the years of the cRIO allowed? We still have ours.

Yes. You could wire a Spark to the PDP, and the output of the Spark goes to the Voltage Converter. That would allow you to turn on/off the voltage converter.

[TimTheGreat]

Originally Posted by rich2202

Yes. You could wire a Spark to the PDP, and the output of the Spark goes to the Voltage Converter.

So we tried connecting the light straight to a talon and limiting the output to 4.5 volts, but it burned out the light. Will that happen with this?

[Peyton Yeung]

Originally Posted by TimTheGreat

So we tried connecting the light straight to a talon and limiting the output to 4.5 volts, but it burned out the light. Will that happen with this?

You should look into how much current is going through the talon at 4.5 volts.

[MrForbes]

LEDs need to have the current to them limited somehow. Adding a resistor in series, will do this.

did you happen to look at the link I provided in my previous post? it kind of explains the resistor thing....

[Alan Anderson]

Originally Posted by TimTheGreat

So we tried connecting the light straight to a talon and limiting the output to 4.5 volts, but it burned out the light.

The output of a Talon is actually either 0 volts or battery voltage. It's pulsed fast enough to yield a variable average voltage, but apparently it's not fast enough to keep the LED from being fried due to too much current.

If the flashlight is happy at 5 volts, then using one of the old 12-to-5-volt converters on the output of a Talon could work. But I'd suggest using a relay (e.g. a Spike) instead of a speed controller, so you never have the chance of making the converter itself unhappy with a PWM'd or reverse polarity input.

[techhelpbb]

Originally Posted by Alan Anderson

The output of a Talon is actually either 0 volts or battery voltage. It's pulsed fast enough to yield a variable average voltage, but apparently it's not fast enough to keep the LED from being fried due to too much current.

If the flashlight is happy at 5 volts, then using one of the old 12-to-5-volt converters on the output of a Talon could work. But I'd suggest using a relay (e.g. a Spike) instead of a speed controller, so you never have the chance of making the converter itself unhappy with a PWM'd or reverse polarity input.

Seconding Alan's suggestion here: I had to read this topic over and over before I realized what people were suggesting. I would seriously think twice about using a FRC style electronic speed control as a power switch in an application like this. As a CSA I'd be looking at that wondering 'why?'. The Spike makes quite a bit more sense, is cheaper and probably smaller.

While it is possible to use PWM to control output voltage, like a switching power supply actually does, it requires a feedback loop and at some point a Spike and a resistor/regulator/converter would have done the job much easier (select the resistance and power rating as necessary to put the LEDs at the right voltage and current, use a series regulator to dissipate the extra voltage as heat or use a DC/DC converter module). Obviously if you use a resistor alone to reduce the voltage to your load the battery voltage will change as the battery discharges. So a series regulator or DC/DC converter may be more elegant but possibly physically heavier and larger.

MrForbes's suggestion was the 7805 series regulator but it wasn't clear if the amount of current required by the light was specified. There are multiple packages for the 7805 depending on the current that will be drawn through it. It may also require a heatsink. A DC/DC converter is an even more complex circuit but the older 12V-5V DC/DC converters for the D-Link are frequently available and provide a healthy amount of current. Just remember those old DC/DC converters drop out if the battery goes below a specific voltage which was around 8VDC.

[MrForbes]

Thanks for expanding on the info I provided. Note that I included a picture, which shows the LM7805 mounted to a piece of aluminum, which is a heat sink. Note also that the heat sink is mounted to a wood part, so it is insulated from the metal robot chassis, as required by the rules...since the metal tab on the LM7805 is connected to the negative battery terminal, through the PDP and Spike