relay driver board project

Posted on Feb 7, 2014

This article means to utilize your old PC to be come a simple controller. Many old PC like 8088 type, 8086, 80286, 80386, or even 80486 already become an obsolete systems. This board using 8-bits of LPT printer parallel data port (DP) to activate the relays. LPT printer parallel port also have 4-bits additional port (PC) which can be utilized too.

Click here to download the full size of the above Circuit.

But this is not implemented in this design, because in my opinion, 8-bits are more than enough, and beside that this size (1 byte) fit each of 8255 PPI port if we want to interface it too. The basic is the same for all the eight channels. To protect the LPT port from damage, I use optocoupler to isolated the PC side (+5V) and relay circuit (+24V). You can change the relay voltage type to +12V or even +6V if you can find one. This type of optocoupler, PC914 can be replaced with another type like 4N series (4N22 ~ 4N28, 4N35, 4N47 ~ 4N49, etc. ). Using a common driver combination, transistor Q1 ~ Q8 (hi-power) to drive the relays. D1 ~ D8 use to protect the transistor from any transient current when the relay is in the off state. The resistor values are chosen to make transistors get full enough saturated when it is on. This common driver also can be change using IC driver like: ULN2002 ~ ULN2004 types. Each package contain 6-bits line drivers. So you need 2 of this IC, but one does not maximize. If you use PC port, than this IC can be utilized. A good alternative. Note that, for the relays, it must be stand the high voltage and current (big power) if you want to use it to drive any equiptment from the AC line. Find a good ones with a high rating output. Careful about the hi-voltage line on the PCB. R1 ~ R16 = 10k. 16 pcs R17 ~ R24, R33 = 1k5. 9 pcs R25 ~ R32 = 1k. 8 pcs C1 = 2200 uF/50V (elco). 1 pcs C2 = 1000 uF/25V...

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