11 May 2010
Author: Giorgos LazaridisA 7-seg pre-wired Breadboard Module
It happens many times, when i design a circuit or a project, to need a number indicator. A few 7-segment digits usually are enough, yet we all know how boring it is to connect wires on the breadboard, and especially small wires. To multiplex for example 5 7-segment digits, i need more than 40 small wires to connect them in parallel. Not to mention the breadboard space that they preserve. And of-course, it is always subject to a bad wire connection or a resistor short-circuit. So, i decided to make this module.
So, the idea is to make a PCB with 5 common cathode 7-segment digits, with all anodes connected in parallel with respect to inputs. The 8 pins from the anodes (a through g plus the dot) along with the 5 cathodes (one from each digit) will be driven to a 13 pin connector. Whenever i want to multiplex up to 5 digits, i will only insert the module on the breadboard, choose the resistors for the connection (if any) and select the transistors (again if any). So, that is the circuit:
In the first design, i had also the resistors on the PCB, as well as the transistors. But, for the sake of flexibility, i considered all these parts as "external components" and the schematic is down to minimum.
Strangely, i made it on single side PCB. One of my guidelines was to have the 13-pin connector a little bit away from the main PCB body, and that NO other PCB route would go lower than this connector. That is because, i wanted the design to preserve as minimum space as possible on the breadboard. Here is the PCB layout (300 dpi image). The red lines can be either the top side (if you use 2-sides PCB), or wire bridges (as i have done):
Then i drilled the pads and did the soldering job. Notice that, the 13-pin connector is soldered from the bottom side! That is because this connector will be directly inserted into the breadboard, while the displays must be facing upwards:
This is the final product under test. As you see, compared to the breadboard, this is minimal, and the extended connector will further save space from the PCB. I strongly suggest you make yourself such a module!
A final touch
The problem with 7-segment displays is with the direct light. If light falls on the display, then it is hard to see the readings. There are special filters for this reason. So, i got one red filter for the job. I used paper tape to mark the dimensions of the LCD module. Using an iron saw (i like this saw because it cuts smooth), i cut to size the filter. I glued the filter on the displays using hot glue. The results are more than good!:
And the result:
A better version from a site reader! (December 13, 2011)
George is an active member of pcbheaven. Last week, he told me that he made this project for himself, but made also some changes. First change is that he added sockets for the displays so that he can use both common anode and common cathode displays. Second change is that he added 2 holes on the top side for two support spacers. And third change is that he made the connector projection a little bit longer, so that the board won't interfere with the power connections of the breadboard.
George sent me also the eagle board files with the updated version. I totally recommend you make this version instead. Thank you George for the changes and for providing us the info.