“This is a project that I have been meaning to finish for over a year now. it is a heart shaped, BEAM based pummer circuit I made to charge up during the day and flash like a heart beating at night. The solar engine is a SIMD1 solar engine by Wilf Rigter and the pummer is a modified version of his power saver flasher which flashes 2 LEDs off the one oscillator.
My final design lasts about 3 hours from when the sun starts to go down.
I personally love creating PCBs as a step up from breadboard or perfboard circuits, and I especially like designing PCBs that go a little beyond just connecting components. Hopefully through this Instructable you will see that while a PCB designer such as Eagle can be superbly powerful, creating a functional and aesthetic circuit such as the one presented here can be quick, fun and really accessible. I spent a day designing it all and only spent around $30USD on the PCB and components! Granted I already had assorted resistors, caps, and a soldering iron.
The first step is to prototype the circuit on a breadboard. I used a design almost exactly the same as Wilf’s pummer from the link in step 1 however I replaced a 100k resistor at the output of the pummer with an LED and added a resistor between the output capacitor and the output LEDs to get a less bright, but longer flash. You can see here in the second photo the waveform at the node between the output LED and resistor in Wilf’s original design. The peak above 2.2v is where the LED lights and the dip below just dumps charge through the 100k resistor. In my revision seen in photos 3 and 4, both the peak above 2.2v as well as the lower peak create a flash (because the voltage is being regulated to ~2.9v by the blue LED, in photo 4 you can see the other 2.2v difference, this time between the 2.9v rail and the node between the output LEDs). This revision does mean that the reference voltage LED does have to be a higher voltage than your output LEDs (and hence the circuit will run for less time) however there is less wasted charge. I like this way better however 2 oscillators, with the second acting as a slave to the first and using something closer to Wilf’s original circuit would also work. Just let me know if I can clarify any of this and excuse the photos of the oscilloscope screen, I had some trouble saving to USB.
As I was almost following WIlf Rigter’s design exactly, I mostly used this step to calculate what value components I was looking at using to make the flashes beat at a heart-like rate.
Once I had the circuit operating as expected I used this circuit to calculate roughly how long my pummer would last once it started flashing at night.
To calculate how long our circuit will last we can use 2 formulas based on the charge stored in, and the charge leaving the storage capacitor.
First we know that Q=CV that is, the charge in the capacitor, “Q”, is equal to the capacitance x the voltage across the capacitor.
We also know the formula for current I=Q/t where “I” is the current, “Q” is charge and “t” is time.
Rearranging these we can get I=(C*ΔV)/Δt where “ΔV” represents the change in voltage and “Δt” change in time (in seconds)
Hence, by measuring the voltage at one point in time and then measuring it again a certain amount of time later (I usually go about half an hour for more accurate results) we know have all the numbers on the right half of the equation and we can see how much current the circuit is drawing from the capacitor on average.
Now knowing “I” and knowing that the capacitor will charge up to about 4.5-5v during the day and shut off at around 2.8v for this blue LED we can calculate the time it will take to go from fully charged to fully discharged. Δt=(C*ΔV)/I
As an example, using the first equation I found this circuit draws around 200uA. Now looking at the second equation and knowing the capacitor is 1 farad,
Δt = (1*(5-2.8)) / (200X10^-6)
Δt = 11000 seconds
Δt = ((8500/60)/60) hours
Δt = 3.06 hours”