Introduction: Solar Engine Demo Using the RFD88A Energy Harvesting Engine Module

About: Microwave Engineer, Consultant, Design Contractor, Electronics Enthusiast, Power Amplifier Designer, Filter Design, Electromagnetic Simulations, Orcad PCB Editor Design

This instructable describes how to make a solar engine demonstration using the RFD88A Energy Harvesting Engine Module with a solar cell, super capacitor and a micro-motor. Solar engine circuits/modules are used when you have a weak power source (solar cell, piezo electric source, RF-DC converter) that needs to drive a system such as a motor or transmitter that needs much more power than the weak power source can deliver continuously. A solar engine acts like a voltage sensitive switch to discharge a capacitor through the circuit. The weak energy source (solar cell in this case) will charge a capacitor and when that capacitor gets fully charged to an upper voltage (VHIGH) the switch will close inside the module and the capacitor will discharge. There are many public domain examples of solar engine circuits for BeamBots. Here at TinkerJim has an excellent writeup of the Easter Solar Engine at

Step 1: RFD88A Module Description & Pin Out

The RFD88A module is similar to these solar engines and has some additional features:
- Very small size 8mm x 10mm (0.31" x 0.39")

- User selectable upper voltage: 5.2V (leave pins 2 & 3 open)/3.0V (pin 2 open, pin 3 grounded)/2.5V (pin 2 grounded)

- Fixed lower voltage of 1.0V

- Easy to use with 4-pin 0.1" DC Headers

The RFD88A module can be purchased at the following website:

Pin Description

Pin 1: Positive DC Input. Energy harvesting input voltage. Also connect the positive pin of the capacitor here.

Pin 2: Leave open for 5.2VHIGH and 3.0VHIGH operation. Ground for 2.5VHIGH level.

Pin 3: Leave open for 5.2VHIGH operation (also Pin 2 must be open). Ground for 3.0VHIGH with Pin 2 open.

Pin 4: Positive DC Output. Connect your motor or higher current circuit here.

Pin 5 & Pin 8: Ground. Connect the other side of the cap and energy harvesting ground to either pin 5 or 8.

Pins 6 & 7: Not used. Leave open or ground.

Step 2: Parts & Tools List

- RFD88A Energy Harvesting Engine Module with two 4-pin headers. The headers come with the order if you request it.

- Solar Cell with an open circuited voltage of ~6V. You can use lower output voltages but this solar cell will allow you to try all three settings of the RFD88A module (5.2V/3.0V/2.5V). More fun!

- Super Capacitor >500mF. Shown is a 1F, 5.0V supercap. Note the polarization of the capacitor and make sure you connect to the correct terminals.

- Micromotor from a hobby shop. I used a BLH3302 Tail Boom Assembly for a helicopter.


Fine tipped soldering iron w/ solder.


Wire. I used wire-wrap wire since it was very flexible.

The sun!

Step 3: Solder the 4-Pin Headers to the RFD88A Module

First you need to solder the 4-pin DC headers that come with the RFD88A module to the backside of the module. With the 4-pin connectors, I pressed them against something hard to make the pins shorter. They will slide through the plastic if you press on them one at a time. Push them until they start to move and then even them up. If you go to far just push them back. This is optional but it made it easier to solder and kept the pins from going beyond the pads.

It helps to hold the module in place with a weight while soldering. Too bad we don't have three hands. Hold the DC header pins with tweezers so you don't burn your fingers. You may want to tin one pad on the module with a dot of solder and then press the iron on top of the pin to melt it in place. As with most ICs and modules, you want to limit how long you solder or you may cause internal damage to the module. Also these white connectors melt when they get too hot so please be quick.

Step 4: Solder the Super Cap

Next solder the super cap as shown. Put the positive terminal to pin 1 on the module and the negative terminal to pin 8 just across from it. I bent the leads first and positioned the cap to make sure it would solder nicely. You may also want to trim the leads to be even since some polarized cap wires will have one longer wire.

Step 5: Connect the Motor and Solar Panel and Voila!

Next solder the wires of the motor to Pin 4 and Pin 5 of the module or you can use alligator clips. Lastly connect the solar cell to the cap wires (pin 1 = red/positive & pin 8 = black/ground). Your solar engine is ready to use. Please note that at first you may also want to ground pin 2 to make the VHIGH setting to be 2.5V. This way you can test your system out at a lower voltage first. Check the specs of your motor to see if it will handle the higher voltage settings of 3.0V if only pin 3 is grounded (pin 2 open) or 5.2V if both pins 2 & 3 are open.

Step 6: Operating Solar Engine

The photo above shows the system with a multimeter connected to the supercap terminals. In bright sun the cap would charge in 2-3 minutes and then the motor would spin the propeller for ~30 seconds before shutting off and recharging again. There are lots of cool projects that can be done with solar engines and it's a fun circuit to work with. The RFD88A module is a building block and makes it easy if you don't want to make your own circuit and want to get right into a fun BeamBot application.