actually, i dont really understand about "the active pull-down device will handle the "flyback" current" thing. can u explain it to me?
tq
In the 555 (regardless of type, CMOS or bipolar) there is a transistor on the output to pull pin 3 high and there is another one to pull pin 3 low.
When pin 3 goes high, it causes current to flow into the relay coil, that current is limited by the coil's resistance and the output voltage of the 555 according to Ohm's Law.
When pin 3 goes low, the magnetic field around the relay coil will collapse, causing current to keep flowing through the coil, and to do this the coil will drive pin 3 toward ground.
If the 555 is an NE555 (with bipolar transistors), the transistor that supplied the current to the realy's coil is an NPN Darlington, the emitter of which is connected to pin 3.
When pin 3 switches low, the emitter of that NPN Darlington will continue to supply current to the realy coil as the magnetic field collapses, thus holding pin 3 near ground.
If the 555 is a CMOS version, then things are as above, except when pin 3 switches to ground, the parasitic diode that is part of the MOSFET that pulls pin 3 low will conduct the current from the relay coil as its magnetic field collapses, thus holding the voltage on pin 3 to a little below ground until the filed has collapsed.
Sorry, but I mistyped "Wiper" in my earlier reply. The wiper of the VR needs to be connected to one end of the VR or the other.