Hi all,
I'm trying to build a cheap one shot relay driver. I had some electronics during my uni years but that's been a while and I'm having fun digging it all up again but I have a few questions I hope someone can answer for me.
I would like a bi-stable relay to be switched by a pulse, even if the input (switch) stays closed a bit longer (let's say 10 seconds). Lots of people tell me to use a 555 but I'd rather use 'simple' components. Not that I can't understand a 555 but I just like this better and it makes me understand the components better. So, let's forget about the 555 OK?
My first circuit looked like this:
The idea being that after closing the switch, C1 has no charge and T1 would be saturated, powering the switch until C1 has so much charge that Vbe would be too small and T1 would close. Once the switch is opened, C1 would recharge through R2.
I had this in a test setup and it worked. A few people had some recommendations which lead me to this changed schematic:
D2 is added to allow C1 to properly discharge. People told me that without it it would only 'leak'. Anybody have any comments on this?
Also for T1 (=bc457) there is a max Vbe of 7V so I added a voltage divider R3 to apply less than 7V to R1/C1/T1.
Another question I have is this: Ic for T1 is Ib*Thfe. But A capacitor does not let DC current through, correct? If the latter is true, then howcome the transistor conducts at all? It's confusing because some people say all it takes is Vbe around 0.6V for T1 to start conducting but they don't mention Ib at all. So is a base current really needed or is a Vbe of around 0.6V all it takes? Or does C1 conduct?
Any comments on my circuit and/or answers to my questions would be much appreciated.
Thanks,
Vincent.