I had a tacho dial left over from my old motorcycle, when I replaced the mechanical rev counter with an electronic panel (that's another project!) and I didn't want to throw it away. These things are designed to be backlit when the bike lights are on so I thought it would make a cool clock. I didn't want to have to switch the lights on manually and I also didn't want to have to regularly change batteries so I thought about making a darkness detecting, solar charging clock.
I considered all kinds of combinations and settled with a higher voltage panel and two higher-capacity batteries, because I want it to just keep working! So far it's been running for several months, lit up all night and charging through the day (it lives on a window sill).
Step 1: Parts
A standard 1 AA battery clock movement kit
A black plastic project box
Some strip board
Molex-type two pin connectors
Double AA mount and 2850 mAh NiMH batteries
A 2N3906 transistor
Nylon stand offs
Assorted resistors and a potentiometer
Some low-voltage white LEDs
A 4V, 150 mA solar panel
Step 2: Circuit
I got the idea for the basic circuit design from here:
The idea is that the battery runs the clock at all times. When it's light the solar panel charges the battery, and when it's dark and the panel stops producing current the backlights come on. Where this gets a bit trickier is that these clock movements run on a single AA, which isn't generally enough oomph to power a LED. I considered a few ways around this - including using a joule-thief type circuit, or putting two AAs in series and tapping off between them to power the clock.
In the end I went for a compromise. I wanted white LEDs and found some units on eBay that have a voltage booster in them already, but they still don't really work off a single AA (they do, but not after the voltage drop across the diode and transistor), put the batteries in series, and connected the clock inline with a resistor to step the voltage down. A resistor is necessary to go to the base of the transistor, I made that a pot so I can adjust the sensitivity of the light-detection part of the system.
In the attached circuit diagram:
(EDIT: realised I had the diode in the wrong place! New version uploaded).
V1 is the 2-AA battery module
V2 is the 4V solar panel
D1 a 1N914 diode
R2 4 K resistor to drop the voltage across the clock
X1 the clock movement
R1 a potentiometer 0 - 5 K
Q1 a 2N3906 transistor
L1 the white low-voltage LEDs
Step 3: Front Face
I used a standard hole cutter on my drill to make the circular opening in the lid of the project enclosure, then drilled two small holes to the side, which mounts a strip of clear perspex across the opening. The tacho dial fits in between the perspex and the lid, and is secured to the perspex with the two small screws you can see (The tacho face has two small holes either side of the central one anyway for mounting the original counter).
These clock movements have a threaded shaft for mounting through things - you can get them with a range of lengths. The tacho face is quite thin but I wanted to leave a gap between the movement and the dial to allow the dial to be illuminated. I got a movement with a longer shaft and stood it back with a spacing nut. The original battery box of the clock is empty, I soldered wires to the tabs with a molex plug on to connect it to the board. I used these plugs for pretty much everything so that I can replace any individual part if I want but it could all just be soldered together of course!
The LEDs were still too big to go between the clock and the dial, so they are mounted at the bottom, pointing up. I stuck tinfoil to the front of the clock movement, and a skirt of it around the sides and top, to spread the light around and hopefully make it more uniform as it backlights the dial. The LEDs were just wired up in series.
The pointers that came with the clock weren't the right colour - I gave them a coat of white primer and then coloured them in with a red marker!
Step 4: Solar Panel
These panels are incredibly cheap on eBay if you can wait a while for them to arrive - and take the claimed spec with a pinch of salt! I think it's safest to assume they aren't as powerful as they say and overspec them for your project.... I build a rig to test their output but that's another story.
I fitted this one with yet another molex plug - they can be a bit nerve-wracking to solder to the back of, these panels, so I'd advise getting a few spares - this one cost me about £1! I made a small mounting frame from thin tin plate with a pair of snips and superglued it to the back - this helps the panel sit at a slightly better angle for catching light.
Step 5: Alternatives
So... if you want to build something like this, an alternative is pulling apart a cheap solar garden light. They often have a neat little IC controller like the YX8018:
I had a look at one of these, and as well as doing the switching logic they contain a voltage booster to run white LEDs. The light I looked at had one low-capacity AAA battery and a feeble 2V solar panel but you could probably use the board with other parts - I thought it was more fun to build mine from scratch though.
I also thought about using a slightly more advanced clock movement that is radio-synchronised - maybe in the next model. Or build a clock movement from scratch? Trickier with a mechanical action but I'm sure someone has done it!
You can also do loads of different kinds of case for the project - I used a black plastic electronics box because it was easy to work and it looks in keeping with the black 1980's motorcycle that I pulled the dial from.
I've also put together a little test rig to connect these cheap solar panels to an ammeter / voltmeter so I can gauge how well they are working. I plan to extend that and make an output from the clock so I can see what current it is drawing both on day and night mode, and then I'll be able to better match the output of the panel to the capacity of the batteries, and the draw of the clock and lights - I want it to work all winter (I think it will as it is, but it's probably over-specified).