Dear Adnan,

It would have been useful if you had mentioned the coil voltages / AC or DC of the relay and solenoid. Anyway this resetting happens on two accounts mainly :

A. The reverse EMF disturbance when switching an inductive load like solenoid / relay

B. The power line of the MCU momentarily falls below the acceptable minimum level when the load is switched ON.

Let us answer some simple questions :

1. Have you wired the freewheel or reverse diode across the relay coil ?

2. If the solenoid is DC type then does it have a freewheel diode across it ?

3. In case the solenoid is AC type then does it have a snubber across it ?

4. If the above are provided then have they been provided near the solenoid / relay and not a few meters away in a junction box ?

5. Are you using different Power supplies for 8051 supply and the solenoid / relay power ?

6. Have you fitted adequate decoupling capacitors on the MCU ppower supply line ?

If you answer "NO" to any or all of the above then your problem could lie there. Just check.

Good luck.

Raghu.