yun relay naman simple lang. Una, hanapin yung 2 terminals na andun ang coil. nasa tabi nakalagay ilang volt coil yun, 6, 12, 24 V. Resistance test....kung zero...hinde yun...closed contacts yan or normally closed. connect the supply dun. You can har a click at kung see through, u can see the contact closing. AYUN!
Now, kung SPST, 3 ang terminals natira. Kung DPDT, eh si apat pa. Well, look for the NO and NC for normally open and normally closed. Yun NO, kapag may supply sa coil magiging closed and vice versa.