It’s a very simple little board, consisting of a driver chip and a relay and the connection to the dot matrix display. I chose to use the ULN2003 to switch the relay primarily because the package already contains a freewheeling diode and can be directly driven from the arduino and secondly because I have 3 tubes of them in the shed already due to a misorder on a previous project. The dot matrix was also used on the same project. It is the 0832 from sure electronics and can be brought pretty cheaply on ebay, for only £7 inc P&P.
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