+------+    +-----+    +--------+        +-------+
 |      |    |     |    |        |        |       |
 |      +--->+ DAC +--->+ Driver +---+--->+ Relay |
 |      |    |     |    |        |   |    |       |
 |      |    +-----+    +--------+   |    +-------+
 | 8051 |                            |
 |      |    +-----+                 |
 |      |    |     |                 |
 |      +<---+ ADC +<----------------+
 |      |    |     |
 +------+    +-----+