Lamp Board

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The blinking light on the top of the RoboMower is handled by a small circuit board containing a light bulb and a resistor. Various models have used different light bulbs; some with a screw-in socket and some with a bayonet socket. All use a 24v bulb and a 20 ohm, 1 watt resistor. Pin 1 on the board is positive.

RoboMower Lamp Board.

There is lots of room in the top light compartment. This could be a good place for hacking in something fun.

RoboMower blinking light compartment with orange lens removed.
RoboMower lamp board modified with LEDs.
Doug Smith: I replaced the light bulb on one of my mowers with two bright white LEDs. I just grabbed parts locally from Radio Shack. The LEDs are part number 276-320, which are rated at 3.6v and 20mA. Doing the math (here's a nice online LED resistor calculator) tells us that for two LEDs we need a 1k ohm, 1 watt resistor, which Radio Shack has as part number 271-153.

There are two reasons I used two LEDs. First, LEDs are very directional. By using two pointed at opposite angles it's easier to see them all around the mower. Second, two LEDs dropped enough voltage to get to a resistor value that Radio Shack had in stock.

I used some solder wick to remove the old light socket and resistor from the board then installed the new components. The short lead of the LED, usually near a flattened side is negative, so that goes in the hole nearest the connector. You can see the circuit trace from that hole going to pin 2. The longer lead of the other LED goes in the other hole left by the light socket. Then the other leads from each LED are soldered together and clipped short.

The results were very good. I would say that the brightness is close to that of the original light bulb. It does have a slightly different color to it and is brighter when viewed from the sides where the LEDS point.

I will modify my next mower light a little differently. Radio Shack parts tend to be quite expensive so I'll order everything elsewhere. I'll also use a wider viewing angle LED, which should appear brighter overall. The B322 from looks like a good candidate with a 120 degree viewing angle at and about 1/10th the Radio Shack LED price. There are also some good choices at super Bright LEDs and Mouser.

The other thing I may do is see if I can raise the LEDs up a little higher. There's plenty of room in the compartment and that should help make the light even more visible.

--Danny Miller 23:50, 29 April 2007 (PDT)

Indeed the 5mm T1 3/4 package shown above has issues with being too focused thus not very bright from off-angles, additionally 2 LEDs is simply not very much light energy total. Also it is far better to use amber LEDs, the amber lens only filters out the other colors a white produces so around 2/3rd the light power is wasted. I have a different LED solution, using amber SuperFlux 4-pin packages. This package is very common for powerful applications and is widely used for LED taillights and such. Digikey Best version is probably part #160-1635-ND since it has the widest light spread at 120 deg. 160-1624-ND or 365-1163-ND are some alternatives but I'd stick with the first one. These bad boys are rated for 70mA and designed for high efficiency! We need 2x 150 ohm resistors, each handles about 0.9W when it's on, 50% of the time so it's under 1/2W average. Still, given the enclosure and there may be some wacky condition which leaves it steady on without flashing, I'm recommending 2W resistors Digikey# OY151KE-ND. 3W's fine too.

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LED board built on 4-pin SuperFlux

As you see I cut a board to fit into the plastic lens itself and scored enough space for 12x of these flat LEDs! There are two series strings, each has 6 LEDs and a resistor and then the two strings are put in parallel. That's Radio Shack's prototype board with the square pads, I used no wire to interconnect the LED pins just globbed enough solder on a pad to bridge to the next square. Even though you need only electrically connect one of the LED's two anodes and one of the two cathodes, be sure to solder all 4 pins to hold it down and add to the heat dissipation. The resistors should be mounted spaced slightly off the board, and in a place where they won't touch the mower's plastic. Instead of modifying the mower's lamp board, I took out the old burned out bulb, worked the glass bulb out of the metal housing and desoldered the wire off the tip of the bayonet base. Thus the bayonet simply plugs into the socket and removing the mower shell isn't even necessary.

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Back of board

I then pushed the + wire off my new board into the bayonet base and soldered it in, and soldered the - to the metal side of the base. So now, bingo! It plugs right in just like the original lamp, the new board sits in the lens and doesn't have room to rattle around or anything.

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LEDs turned on in daylight

It's crazy bright. It actually draws slightly MORE current than the original bulb! The light spread is wide and its light is quite visible even when you're viewing directly from the side. This pic was taken in full Texas sun. It's comparable to a traffic light. It will blind you in the dark so look away when you put in the batt pack. When running at night, it lights up the surrounding trees. Necessary? Not really. But the brightness and its sharp color get some extra notice so I dig it.