When I discovered circuit sculpture, I instantly fell in love with it. Inspired by Mohit Bhoite's wonderful art pieces, I finally decided to build my own sculpture.
For the model, I chose the COSMO-SkyMed satellites. COSMO-SkyMed is a constellation of four SAR (synthetic aperture radar) satellites for Earth observation (it can provide very high-resolution images; there is also a second generation of four other satellites currently being deployed, externally very similar to the first generation).
I decided to use two red LEDs to imitate the microwaves coming out of the large electronically steerable antenna (the best approximation of microwaves in the visible band, I guess).
As the satellite was intended for display on my office desk, I had three main requirements:
- active during working hours
- able to operate in a room, not necessarily close to the window
- output light visible in typical office lighting conditions
In particular, I wanted the LEDs to illuminate the desk for a couple of seconds.
Additionally, I wanted to use components from my parts bin, preferably discrete components for an old-style look.
In a nutshell, I needed a circuit that could collect solar energy and act as a monostable multivibrator that would self-trigger when enough energy had been stored in the "tanks" (two 3300uF capacitors I had in my spare parts bin). To this end, I modified a classic two-transistor latch circuit to make it measure the supply voltage and use its value as a trigger. I also added a capacitor on the feedback line of the circuit to make it switch off after a predefined time (monostable behaviour). I chose the capacitance of the tanks to ensure that the LEDs were fully on (20 mA current) for about two seconds.
The circuit can operate as long as the local light conditions allow the two solar cells (connected in series to maximise voltage) to generate at least 4 volts as open circuit voltage (i.e. with the capacitors charged, considering that the latch circuit consumes very little energy in the 'off' state). As the circuit needs to wait for the capacitors to charge up, the time between two "irradiations" will change wildly according to the ambient light. I like to think of this as a feature, not a bug.
You can find the circuit diagram in the photo gallery or play with it on Falstad (link to simulation). The Falstad file is also available as a project file.
As for the model size, my major constraint was the size of the only solar cells I could find readily available on Amazon. They were 34.5 x 67.5 mm, so the satellite would have looked a bit stubby. However, I thought it was ok for the "unrealistic" model I had in mind. Then, I played with the design on Tinkercad until I was satisfied with the final look. You can see my prototype here.
I decided to place the tanks at the top, near the power source (the panels), and to dedicate the lower 'floor' of the satellite to its brain (the circuit) and transducers (the LEDs). In short, I put the "platform" (or bus) at the top of the model and the "payload" at the bottom - more or less like the real thing.
After designing and testing the circuit the day before, it took me another full day of work to put the satellite together. I hand-draw the templates on a squared notebook as a guide to creating the satellite components. With the help of pliers, wire cutters and a soldering iron, it was pretty quick and fun (even more than expected!). Thanks again to Mohit Bhoite for his precious tips.
Why is this model environmentally friendly? Apart from the solar cells and the brass rod, all parts were recycled from my personal collection of e-waste.
UPDATE: I overestimated the light conditions in my office. In my case, I had to reduce the trigger voltage by shorting the red LED to enable the blinking. The trigger voltage is now defined by the transistor's VBE plus the voltage drop on the yellow LED. In this configuration, the blinking period varies from ten seconds and...Read more »
I absolutely love this!