BEP Application: Pummer

Dave Hrynkiw
July 12, 2007

BEP01-F01-Complete Pummer-DSCN0856.jpg

The following instructions detail how to build a simple pummer using a BC2 (Bicore) BEP (Bicore Experimenter PCB). A pummer is a solar powered self activating LED flasher. The difference between a pummer and a BLFNAR (blinky light for no apparent reason) is that a pummer has a soft turn off, which means that it turns on fast, but slowly fades away.



Theory of operation (How does it work?)

The core of this circuit is an oscillator know as the Bicore. The Bicore just oscillates back and forth with complementary outputs (when one output is high the other goes low). This works to create a charge pump effect to double the voltage sent to the LED, that means off of a 2.4V supply you can flash white or blue LED's that usually require 3V + to turn on.

The charge doubling effect works like this:
On one side of the Bicore you have the negative side of the 1000µF capacitor and on the other side of the Bicore is the negative side of the LED. The positive side of the capacitor is connected to the positive side of the LED and also to a 1K resistor connected to positive on the power supply. When the Bicore is low on the capacitor side the capacitor will charge through the 1K resistor and the LED will be off (the LED will have no voltage across it). Then when the Bicore flips states the LED is forward bias and the capacitor gets electrically attached in series with the voltage supply, the LED will now see the voltage of the supply + the capacitor voltage.

How the pummer automatically starts is by clever use of the enable line. There is a 100K resistor from the enable line to ground (keep in mind that the chip is enabled when this line is pulled low). The solar cell charges the battery through the diode and keeps the enable line tied high if there is sufficient light falling on the cell. When the light level falls below the threshold set by the 100K resistor the enable line is pulled low by the 100K resistor and the pummer turns on.

Circuit Diagrams:

BEP01-Pummer circuit.jpg This is the circuit we will be using to construct the pummer.
BEP01-Dual Pummer.jpg If you want to try it out, here is a circuit for a dual LED pummer. Only LED 2 will see the voltage doubling effect so LED 1 must operate at or below 2.4V to actually light up (green, red or yellow should be fine).
BEP01-Standard pummer.jpg This is a single LED pummer that operates down to lower voltages and can be run at high frequencies to make a flashlight. The disadvantage being that there are high current spikes when the LED flashes and when the capacitor charges, leading to wasted current, and shortened battery life.
BEP01-Technical pummer.jpg This is the technical version of the pummer we will be constructing. One thing that may be confusing is the line running through the inverters, this is the enable line.
BEP01-Lexicon pummer.jpg This is the simplified lexicon version of the pummer. This notation is only relative to BEAM circuits and is not widely used. It does make sketching out a BEAM style circuit quick and easy though.

Construction procedure:

BEP01-F02-Pummer parts-DSCN0764.jpg 1. Gather all your parts:Click here to have these parts added to your cart.Parts list:1 - 74AC240
2 - 0.22µF cap (0.01µF or 0.47µF will work also)
2 - AAA Ni-Cad batteries (or 10F gold cap)
1 - Dual AAA battery holder
1 - Solar panel capable of 3.0V at 20mA (the SC2433 is near ideal)
1 - 1000µF capacitor (or 3300uF for a longer fade away)
1 - Diode, this can be a plain silicon diode but it helps to use a low voltage type diode such as the 1N5818 schottky, or a germanium diode of some sort.
1 - LED, Red, green, blue, white or any color you want to use
1 - 1/16 bronze rod or paperclip or any structural material you want to use
1- 1K resistor (K = Kilo = 1000)
1 - 4.7M resistor (M = Mega = 1 000 000) (or around this value such as 6.8M or 3.6M)
2 - Optional sip sockets
BEP01-F03-Chip and caps-DSCN0767.jpg 2. First solder in the 74AC240 chip, make sure it goes in the right way. Also solder in the 0.22µF caps in the spots marked C1 and C2.
BEP01-F04-Cut trace-DSCN0773.jpg 3. Cut the trace that attaches the enables to ground with a knife, make sure this line is cut or the pummer will not shut off.
BEP01-F05-Ad 100K resistor-DSCN0779.jpg 4. Add the 100K resistor in the spot labeled R1, this is right underneath the spot where the trace was cut.
BEP01-F06-Add diode-DSCN0782.jpg 5. Now solder the diode in place, Cathode goes to positive and Anode goes to the 74AC240 enable line, note that the pads the diode is soldered to are free they still need to be solder bridged to the enable line, we take care of that in the next step.
BEP01-F07-Solder bridge-DSCN0789.jpg 6. Run a solder bridge between the free pads and the enable line pads, this connects the diode Anode to the enable line.
BEP01-F08-1000uF cap-DSCN0790.jpgBEP01-F09-1000uF cap 2-DSCN0795.jpg 7. Next solder in the 1000µF cap, the negative side goes to the output labeled O1. Take care that the other side of the capacitor (the positive anode side) this gets soldered to a free pad. We connected the capacitor anode to the pad labeled L2.
BEP01-F10-Add 1K resistor-DSCN0798.jpg 8. Solder the 1K resistor from the + pad to the bottom pad on R3
BEP01-F11-Add Sockets-DSCN0806.jpg 9. Now add sockets in place of R4. You could solder the resistor in and not use the sockets but its nice to be able to tweak later...
BEP01-F12-Solder Battery pack-DSCN0812.jpg 10. Next mount the BC2 board on the bottom on the battery pack with glue or sticky tape. Solder the Negative lead from the battery pack to a ground pad on the BC2 board. The positive lead will get soldered to the positive pad, in this case the same one that the 1K resistor is soldered to.
BEP01-F13- Solar panel sticky-DSCN0819.jpg 11. Solder leads onto the solar panel, red goes to positive, black to negative, and bishop takes the knight. Also added is some sticky tape to mount the solar panel in the next step.
BEP01-F14-Solar panel mounted-DSCN0821.jpgBEP01-F15-Solar panel wired-DSCN0824.jpg 12. Mount the panel on the BC2 board, and wire the negative to any ground pad and the positive to the enable line.
BEP01-F16-Bar to output-DSCN0833.jpgBEP01-F18-Attach second bar-DSCN0849.jpg 13. The only thing missing is the LED. I got a bit creative with some bronze rod, but you can use whatever you wish, or just mount the LED directly to the BC2 board. The flat side of the LED (shorter leg, aka the cathode) get soldered to the output opposite to the 1000µF capacitor (labeled O2). The other side of the LED gets soldered to the junction of the 1K resistor and the 1000uF cap.
BEP01-F19-Attach LED-DSCN0855.jpg 14. Once the LED is in place add the batteries, let it charge for a bit (a couple minuets is sunlight) and try covering the solar cell. The LED should start flashing after a couple of seconds.
BEP01-F01-Complete Pummer-DSCN0856.jpg 15. Have fun with the design. Unlike most projects, the shape and size of the pummer are of no consequence. Get creative with the packaging!


  • Double check that the chip, LED and capacitor are in the right way.

  • Never at any time attach this circuit to a voltage supply of greater than 6V, this will fry the chip.

  • If you have a multimeter, check that there is some voltage in the batteries. Minimal operation is around 1.4V, depending on the voltage needed to power the LED (white or blue LED's may need about 2.0V).

  • The pummer will not turn on until the light level is below the set threshold. If you want to force it on, either short circuit the solar cell or short circuit the 100K resistor.

  • In this circuit, the frequency must be slow enough that the 1000µF cap is able to fully charge between illuminations. Generally, the pummer should not flash more than once a second. If you want a faster flash rate, try using the other single LED pummer.


Hints, tips and useful advice :

  • Playing with the resistors: By raising the value of the 100K resistor tying the enables to ground, the pummer will wait until it gets darker to turn on. By raising the 4.7M resistor, the pummer will flash slower but with more powerful bursts.

  • If you want a really efficient long lasting pummer, replace the 74AC240 with a 74HCT240, and slow the frequency way down. Run only one LED and replace the 1K resistor with a 4.7K resistor. That should run a week or two off of a single charge!

  • We have experimented with different power storage devices and to date, the Ni-Cd batteries still come out on top. A close second is a large 10F 2.5V gold cap. I have some pummers built with AAA Ni-Cds that are still working after 4 years of operation.

  • Pummers can live outdoors. The limiting factor is the temperature range that the power-storage device will operate at. It is feasible to have a post-mounted pummer on the side of a road in the middle of winter, if you bury the battery underground to protect it from getting too cold. If you do want to try this, you must also waterproof the pummer by potting it or sealing it in a container.

  • An outdoor test pummer using Ni-Cds has been running two years, and survives temperature extremes from -30 deg C to +30 deg C. So far, so good!

  • A good place to buy super bright LED's (if price is no object) -

  • You may notice our tendency to put the LED on a long neck. This is to get the LED visible above a window ledge.

Other pummers for inspiration:

BEP01-Pummer trio-DSCN1065.jpg A trio of BEP pummers, the one on the right is the documented one with a spring neck. The one on the far left was built Dec 18, 2001 and uses the less efficient pummer circuit. The spring neck is a bit more robust than a solid one but as long as you don't drop it too much it should be fine.
BEP01-gold pummer 2.jpg This is one of the first pummers built using Mark Tilden's SMT Bicore boards. Built in Aug 1999 and still working fine!
BEP01-Pummers1.jpg A pair of pummer Mark Tilden built, We suspect the one on the left is more of a voltage triggered pummer that works like a solar engine, and the one on the right is similar to a regular pummer.
BEP01-Bright Blue-DSCN1070.jpg A bright blue pummer using Ni-MH batteries instead of Ni-Cds, seems to work fine but needs a bit stronger light to charge. This one can be seen from more than a kilometre away!
BEP01-Small bright green-DSCN1077.jpg This pummer fits mostly under a solar cell measuring 24mm by 22mm (a bit bigger than a quarter) and will run for a little over an hour. The LED is an ultrabright turquoise color.

Comments questions? Email us!

© Solarbotics, 2002


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