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# Timer circuit

M

#### Martik

Jan 1, 1970
0
I want to build a timer, limited to 5 hrs, for my NiCad charger which runs
on 120VAC. I understand that a 555 IC and a properly sized capacitor and
resistor will provide the timing. How do I connect this to AC? Relay? Anyone
have a schematic? Maybe it would be easier to build a decent charger that
won't damage the batteries?

P

#### petrus bitbyter

Jan 1, 1970
0
Martik said:
I want to build a timer, limited to 5 hrs, for my NiCad charger which runs
on 120VAC. I understand that a 555 IC and a properly sized capacitor and
resistor will provide the timing. How do I connect this to AC? Relay?
Anyone have a schematic? Maybe it would be easier to build a decent charger
that won't damage the batteries?

Five hours is much too long for a standard 555 IC. You'd better buy a time
switch. The simple mechanical ones can be set by the quarter. Switch it on
by hand and place an off stop only at the time you want to swich off.

petrus bitbyter

A

#### Alan Harriman

Jan 1, 1970
0
I want to build a timer, limited to 5 hrs, for my NiCad charger which runs
on 120VAC. I understand that a 555 IC and a properly sized capacitor and
resistor will provide the timing. How do I connect this to AC? Relay? Anyone
have a schematic? Maybe it would be easier to build a decent charger that
won't damage the batteries?

Why not just use an appliance timer? Not the most high tech, but quite
effective. See Red Scholefield's "R/C Battery Clinic" for lots of great info on
batteries and charging.

http://www.rcbatteryclinic.com

Alan Harriman

M

#### Martik

Jan 1, 1970
0
Problem is I have to keep changing the stop time, I've seen timers that have
fixed times like 1 2 3 hours but they are $25 K #### Kevin R Jan 1, 1970 0 buy one a 24 hour normal plug in timer with pins you move, take out the on pins and only put a off pin at the required time Kevin N #### NSM Jan 1, 1970 0 Martik said: Problem is I have to keep changing the stop time, I've seen timers that have fixed times like 1 2 3 hours but they are$25

I got one that has little pins to select on and off to the half hour. It was
free after rebate.

N

J

#### James Sweet

Jan 1, 1970
0
Martik said:
Problem is I have to keep changing the stop time, I've seen timers that have
fixed times like 1 2 3 hours but they are \$25

Wiring a 555 timer circuit to a relay is trivial, just use a small
transistor like a 2N2222 and connect the output of the 555 through a
resistor to the base, ground the emitter, connect the collector to one side
of the relay coil and connect the other side of the coil to the power supply
for the timer. A diode backwards across the coil to absorb spikes is a good
idea too. You can also get solid state relays that you should be able to
drive directly from the output pin of a 555.

R

#### Ross Herbert

Jan 1, 1970
0
I want to build a timer, limited to 5 hrs, for my NiCad charger which runs
on 120VAC. I understand that a 555 IC and a properly sized capacitor and
resistor will provide the timing. How do I connect this to AC? Relay? Anyone
have a schematic? Maybe it would be easier to build a decent charger that
won't damage the batteries?

As petrus has said, 5 hours is far too long for a 555 to handle. In
order to get LONG delays you need to use some sort of a counter
circuit. Have a look at
http://www.williamson-labs.com/555-circuits.htm 4th last circuit near
bottom.

M

#### Martik

Jan 1, 1970
0
James Sweet said:
Wiring a 555 timer circuit to a relay is trivial, just use a small
transistor like a 2N2222 and connect the output of the 555 through a
resistor to the base, ground the emitter, connect the collector to one
side
of the relay coil and connect the other side of the coil to the power
supply
for the timer. A diode backwards across the coil to absorb spikes is a
good
idea too. You can also get solid state relays that you should be able to
drive directly from the output pin of a 555.
Thanks, I'll check it out

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