⬆️ | Introduction | Parts Required | Assembly
The Public Rad.io is a single station FM Tuner in a Mason jar open source project by Zach Dunham and Spencer Wright crowdfunded on Kickstarter into being. Their blog discusses how they used the mason jar just because they had one handy and needed to test the speakers for their radio design, but the general benefit is that it in choosing an established standard for packaging goods, since 1858, it is easy to change the container for the radio. The public rad.io isn't actually a single station radio, it is possible for a user to change the station by unscrewing the jar and hitting the seek button. Few listeners are actually station-hoppers, though: most prefer to return to their favourite station and programming!
- The Public Rad.io is available from their website, or the plans for manufacturing them are on Github. I would recommend getting an extra antenna, as the longer mason jar is more prone to tipping and the antenna usually suffers. The power regulator on the board is well designed, it will cut off power to the radio to keep the hybrid supercapacitors from discharging past 2.2V.
- 120x60 3.5V solar panel - produces about 4.2 to 4.3 V open circuit (no load)
- Schottky Diode e.g. IN4004 - this causes a voltage drop of only 0.3 - 0.4V and reduces the voltage from the panel's open circuit 4.2V to 3.9 or 3.8V, protecting the hybrid supercapacitor from overcharging. The diode also prevents the hybrid supercapacitor charge from flowing back into the solar panel when it's not in daylight.
- Electronic protoboard like the Adafruit Full Size PCB. We will use the busses that run along the length of the boards to wire the hybrid supercapacitors/lithium ion capacitors in parallel. The combined capacitance from parallel capacitors is additive, so 6600 Farads.
- ~1000 Farad Hybrid Supercapacitors also known as Lithium Ion Capacitors - available from cds of Taiwan via Aliexpress or Eaton Electronics via Farnell in large sizes.
- a large wide-mouth mason jar, like the Bernadin 750 mL capacity that I use
We begin by removing the AA battery clips and soldering in a positive red wire and a negative black wire. Although this radio uses a small amount of current, using large wires with low power sources has been shown to have a benefit and it supports the longevity of the device.
The solar cell we will use is this one, it is rated for 3.5V but in full sun it will produce 4.2V. When solar cells are used to charge supercapacitors, they always charge to the open circuit voltage, not the rated voltage, due to the lack of resistance which capacitors offer compared with batteries. Solder a wire to the positive and one to the negative - although this photo illustrates the ends, if you use the middle copper circles, the wires can stay twisted and neat (you may be using salvaged ethernet or phone wire).
The purple cylinders are hybrid supercapacitors made by cds of Taiwan. Eaton electronics will have 1400F capacitors out by 2025 (thanks, Niall!).
Each hybrid supercapacitor stores about 6 000 J as it charges to 4V then discharges to 2.2V.
Joules = 1/2 C V ^2 (Charged) - 1/2 C V ^2 (Discharged) -
0.5 x 1100F x 4V^2 - 0.5 x 1100F x 2.2V^2
8800 - 2682 J
6118 J
Hybrid supercapacitors ship fully charged, usually with the long foot (positive voltage) in a plastic tube. As they can discharge MUCH faster than batteries when they short circuit, it is important in handling them to take special care as a significant heating occurs across the leads for anything conductive. A short circuited hybrid supercapacitor will spontaneous melt a line of solder, or make a wire glow red and melt the plastic insulation for example. So proper handling! Doesn't go in a pocket with change, for example. :)
A single 1100F hybrid supercapacitor will power The Public Rad.io for about 15h at medium volume. Having six of them wired in parallel will ensure listening for an entire week without a sunny days with the opportunity to recharge from the panel.
To avoid short circuiting, is useful to solder all the negative leads, clip the steel legs out of the way, then solder the positive leads. Then gradually, one at a time, attach jumper wires from the positive lead-row to the power bus that runs the length of the board. The diode positive end goes into the positive bus, where the supply is the solar panel. In the photo, observe the red wire from the radio attaches directly to the positive power bus, while the black ground wire goes to the ground bus.
Since the volume control is actually a variable resistor, it is useful to turn the radio on slowly, i.e. with zero volume at first, as the hybrid supercapacitors can produce a significant surge, before slowly advancing the radio to the desired volume. Alternatively, you can add a negative temperature coefficient (NTC) thermistor, this is a black disk resistor that absorbs current surges, then as it heats up the resistance drops to zero.