I'm trying to make this simple code lock circuit, and I ordered this keyboard for it.
However, there isn't any kind of manual for it, and I simply don't understand how it works.
Here's a larger picture of it (sorry for the horrible soldering, I've a bad solder, and I probably do it wrong also.)
So there's 7 wires on it currently, but how do I actually use this thing?
There are four rows and three columns which make a matrix of 12 possible connections. Each switch is located at one of the cross-points of the columns and rows: -
If you "read" the value of the voltages on a single column and activate each row with a positive voltage (not together but in turn) you can deduce which of the four switches is pressed in that column.
Read the voltage for each of the three columns and you can deduce which button is pressed of the 12. Two buttons being pressed can confuse things so be aware of that.
+ power the pin 1 (see the picture above), and started to try out the pins on the LED. So the LED has - on the catode. I connected pin 2 on the anode of the LED, and started pressing buttons. Number 2 worked. Pin 3 did nothing, no matter of what button I pressed. So I tested them all, and I managed to light the LED with keys 2,5,8 & 0. With power on the pin 1.
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Most likely the keys are arranged in a matrix, which explains 7 wires nicely. There is one wire for each column and one wire for each row. The keyboard may be as primitive as each key simply shorting its row and column lines together. Or, it could connect them with a diode.
Probe around with a ohmmeter and you should be able to figure out what column and row each lead is connected to. Or, try probing with a 5 V supply, LED, and 2.7 kΩ (about) resistor in series. That will put 1-2 mA thru the led when the two ends are shorted. That will be dim, but should still be visible in normal office lighting. Whatever is in the keypad should not be hurt by 5 V or 2 mA.
+ to one of the wires, - to the another end of the LED and one wire to the anode of the LED and to the keyboard itself?
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If you decide to use an MCU to scan the keypad, below is a polling algorithm that you can use (which I extracted from this source). It took me a while to understand it (had to read it twice), but once I did, it helped me A LOT.
Continuous Polling Operations
In this mode of operation, the MicroConverter continuously polls the keypad for a key press. This operation is used where the MicroConverter has completed a task and is now waiting for input before proceeding. In this mode, the keypad is connected to one port of the MicroConverter, Port 2 in this example. Figure 3 shows the connectivity. The output from the MicroConverter, following a key press, is viewed using HyperTerminal running on a PC. The MicroConverter is connected to the PC via the COM1 port. This is the reason for showing the RS-232 connection.
As can be seen in Figure 3, the four columns (X1 to X4) are pulled up to VDD and are also connected to four of the MicroConverter port pins (P2.4 to P2.7). The four ADuC8xx rows (Y1 to Y2) are connected to the other four port pins (P2.0 to P2.3). The MicroConverter outputs 0 or drives low the keypad rows (P2.0 to P2.3) one at a time and checks the columns (P2.4 to P2.7) for a low condition.
For example, the following is the sequence of events up to a switch press detection (Switch 5 in this case). The MicroConverter outputs a low on P2.0 (Y1) and checks for a low on P2.4 to P2.7. In this case, no low is found and so it returns P2.0 (Y1) to high and moves on to P2.1 (Y2). The MicroConverter now drives P2.1 (Y2) low and again checks P2.4 to P2.7 for a low condition. This time it finds that P2.5 (X2) is low, due to Switch 5 being pressed. The MicroConverter now knows that the interconnect of Y2 and X2 has been shorted, therefore, this is 5.
If you have an Arduino handy, you don't need to implement the algorithm yourself. Instead you can use the Arduino Keypad lib. The links are below:
See more details (on how the keypad is internally wired, for example) here in my other related answer
