I am trying to design a 30 second timer circuit to switch an automotive solenoid that draws about 0.8 amps, and need some help please.
StackExchange posts have been very very helpful so far, and by reading older posts it has helped improve the design of the circuit with protection diodes D1 and D2, and decoupling capacitors C3 and C4, placed very close to pins 1 and 8 of the 555.
Due to the configuration my control circuit must be above the load, so I am using a PNP transistor (Q2) to switch the solenoid. The Base of the PNP is connected to ground through an NPN transistor (Q1), which is switched by the output of a 555 chip.
My problem is that the solenoid switches on as soon as the NPN transistor Q1 is connected. Disconnecting either the 555 output or the connection from the transistor to earth switches the solenoid off.
When prototyping, it worked previously when using BD682 as the PNP transistor and BD681 as the NPN transistor with appropriately sized base resistors, but have now switched to surface mount parts and is broken. Any advice gratefully received!
In this thread: PNP to NPN Transistor switch, some of the replies discuss adding transistors between the Base and Emitter of the PNP transistor due to leakage of the NPN transistor, but R5 does that in my circuit, I think?