# Why does a relay open gradually when driven by 555 through an emitter follower?

A follow-up to my earlier post Why is the output from this astable 555 the reverse of it's simulation?

The 555 output drives a 2N2222. The 2222 emitter-follower with a 4.7 kΩ resistance to ground, in it's turn, attempts to drive a PCB mountable relay (I meant to drive a 3055 and then the relay but decided to try to drive the relay directly before including the 3055). To determine whether the relay is triggered an analogue multimeter (resistance) has it's leads applied to the contacts on the relay.

This is curiosity rather than a problem; when the 555 output goes high the relay makes a lot of noise, and the NC opens over around 10 seconds from 0 Ω resistance to around 1 kΩ slowing as it increases (some kind of logarithmic rise).

• VCC = 12 V
• Rb at 2N2222 = 330 Ω
• Re at 2N2222 = 4.7 kΩ

EDIT: The relay JQC-3FC is connected at the take-off above R3 with the other end of the relay going to ground.

Applying 12 V to the relay directly switches the contacts instantaneously. Why the gradual opening through the timer circuit?

• Exactly where is the relay coil connected? Across the 4k7, between the collector and +12V – JIm Dearden Jul 6 '13 at 10:54
• @JImDearden: At the top of the 4k7 with the other end going to ground. – Everyone Jul 6 '13 at 10:58
• what is the current rating for the relay coil? – JIm Dearden Jul 6 '13 at 11:07
• Way too much handwaving. This needs to be closed until a proper schematic is supplied. – Olin Lathrop Jul 6 '13 at 12:59
• When you say the relay is making noise it makes me think that something is oscillating. Why are you using an emitter follower to drive the relay, instead of an open collector? Why the resistor in parallel with the relay coil? – jwygralak67 Jul 10 '13 at 17:29