# Confusion with 555 transistor relay driver

This is probably schoolboy stuff, but before I construct it ...

A 555 will output almost the supply rail, or down to zero. A transistor attached to that, with a relay on collector can drive a 12v relay.

My basic question is Does base voltage affect CE voltage of a PNP ?

If the high output of the 555 is 12v, there is no voltage drop across CB, so how does the 12v coil - connected to Vcc and collector - energise? (One simulator suggested that with a base drive of 11v, the collector would be at 11v2, just 0.8v below supply rail ???)

I've also got into a 'corner' on another part of the circuit. A 4017 is running at 6v, so the pin out is around 6v. But ... I want to link a high out from the "9" output to the threshold pin of the 12v 555. As can see, the 6v is less than the 8v threshold I need. My idea (so far) -again, want to check before spend a couple of hours soldering- is shown below. When low, current will travel through R3 (22K) on emitter, through base/diode/resistor (r2:10K) This should pull R3/threshold lower (R2+R3 potential divider to give around 4v) When the 4017 goes high, Q1 will not conduct as diode will be reverse biased and R1 should go high

** First attempt at schematic. The 1htz is actually the 4017 output at 6v that goes high after 5 mins, and the voltmeter the threshold input that should - in theory - go between 4v and 12v, (although batt is shown as 9v in diagram)

simulate this circuit – Schematic created using CircuitLab

The 555 is actually wired as a Scmitt trigger. A resistor / capacitor triggers the 555 at switch on, the output turning on the relay. After 5 mins of 4017 timing, the "9" output counter supplies 8v+ to the threshold, turning off the relay.

• can you add where exactly your relay coil is in your schematic? (I'd also ask about how the relay coil is, but that'd be too philosophical, maybe) – Marcus Müller Jan 18 at 10:49
• If the square wave is 12V, the diode is nonconducting, and the base floats, no? – Scott Seidman Jan 18 at 11:28