My love for kits and cheap thrills comes together when I shop online. A year-or-so ago, I scoured eBay for whatever cheap kits I could find so I could build them and given the rainy weather, it is time for another one.
This one wasn’t well described but it is another “generic” kit which is claimed to be an electronic three-person voting kit. It cost just AU$1.85 including postage, which is incidentally cheaper than the cost of sending the kit across Australia as a large letter which costs AU$2.00. How could I lose?
It seems that even a zip-lock bag is too expensive for this kit, so it comes in a regular plastic bag secured with adhesive tape.
Once opening the kit, there is a piece of paper with a bill of materials, useful schematic and a Chinese description of what the kit does. There is the PCB and loose components including four resistors, three switches, two ICs including sockets, a capacitor, an LED and two header pins. The kit really doesn’t have much to it.
The PCB is a fibreglass type with silkscreening on the top without any soldermask. The drilling seems to have been done rather bluntly, so the fibreglass substrate seems to have been “torn” slightly around the holes. The edges of the board are a little rough.
The pattern is single sided – all on the rear. Two of the holes have no donut pads on them, which appears to be a mistake as other not-connected pins still have a donut pad. There is solder resist and the finish is a tin-plating making it easier to solder, although soldermask alignment is slightly off. It would be nicer if the PCB had thermal-isolation for the pads to make them slightly easier to solder given the cramped nature of the pins – especially around the switches which could encourage solder bridges.
Construction and Functionality
Lets start with a quick calculation of how many solder joints are needed:
U1 - 12 (due to pad errors) U2 - 14 4 Resistors - 8 3 Switches - 12 LED - 2 Capacitor - 2 Header Pins - 2 TOTAL - 52 joints
The kit itself is very straightforward with no instructions necessary. Shove the components in the right holes and solder away. Finally put in the ICs and we’re ready to rock and roll.
I found soldering fairly easy, with the finish on the PCB being easily wet with solder. The switch pins were slightly close for comfort – those without fine-tipped irons or solder may find it easy to create solder bridges.
As for what the kit actually does – it’s pretty simple. Supply 5V (no reverse polarity protection so take care to follow the markings) and the LED lights if two or more buttons are pressed. That’s all there is to it.
Quickly tracing how this is done – the switch and 470 ohm resistor form an input (A, B, C) to the first quad NAND gate. This computes /AB, /AC and /BC. The outputs are then fed into a single three input NAND gate, thus performing the function /(/AB./AC./BC). Using De Morgan’s Laws, this can be simplified to AB + AC + BC, as expected.
The kit itself is really not much to look forward to, although for the price, I suppose that’s not unexpected. It’s a simple demonstration of cascaded logic built using two-input and three-input NAND gates with one output displayed on an LED. The circuit itself basically implements AB + AC + BC as a logical function, so there’s really nothing unusual in what it does. It’s not difficult to construct on a decent quality PCB with included IC sockets, although two pads are missing from the PCB.
In all, I think this is a kit that could be passed up – the only thing it reminded me of was De Morgan’s Laws – break the line, change the sign!