From BEAM Robotics Wiki
 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.
 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!
- You may notice our tendency to put the LED on a long neck. This is to get the LED visible above a window ledge.
- 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.
 Alternative Diagrams
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. It mixes electrical symbols and symbols of Biomorphic Maps.
 Alternative Versions
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).
 Low Voltage Variant
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.
 Improved Version
Wilf Rigter pointed out the following problem with the standard BEP Pummer circuit:
- One thing about the schematic you used is that there is a single 4.7M resistor between input pin 2 and input pin 17. In later versions of the circuit that resistor is replaced with two 4.7M resistors. One is connected between pin 2 and pin 20, The other is connected between pin 17 and pin 20. The reason is that the single resistor version allows those inputs to "float" during charging and that can cause the chip current to rise and drain the batteries.
He provided the following schematic to address these problems:
 Choice of IC
Wilf Rigter has this to say about the choice of IC for this circuit: In particular, the 74AC240 has 4x the available output current of a 74HC240. As the LED current is limited mostly by the CMOS output resistance, this translates into somewhat brighter flashes. Keep in mind that at low Vdd (2V), both logic families operate with logic levels barely exceeding the mosfet gate thresholds and with low transconductance. By the same token, the 74AC quiescent supply current is greater than HC, especially when the input logic level is near the input switching threshold. There are other considerations when comparing HC vs HCT logic families. These have different input logic thresholds. The lower threshold of HCT (or ACT) will increase the time constant of a grounded bicore as used in the pummer application but decrease the time constant of a Vdd referenced bicore. This may be exploited in some applications, for example, you get a two speed (4:1) oscillator by switching the common of the grounded bicore timing resistors between ground and Vdd.