# Countdown timer switch

Discussion in 'The Projects Forum' started by jimwells41, Jan 7, 2009.

1. ### jimwells41 Thread Starter New Member

Jan 7, 2009
1
0
I need a countdown timer that will switch 110v 1a.
The timer range should be up to 12 hours.
This is to be used to feed the charger for the battery of my electric bicycle. They say I should not let the charger operate for more than 12 hours.
So ... i would come home, connect the charger to the battery, set the timer and forget it, as opposed to worrying about disconnecting the charger.
I see wall mounted devices that will do the job, but \$70 seems excessive. Any suggestions about dealing with this would be appreciated.

Jim

2. ### mik3 Senior Member

Feb 4, 2008
4,846
69
Use a 4047 timer IC and a relay to control your charger.

3. ### beenthere Retired Moderator

Apr 20, 2004
15,815
293
I have a cheapo timer I picked up a Wal-Mart for about \$5. You can set on or off periods for up to 23 3/4 hours in 15 minute increments. That should be enough to turn off a charger.

4. ### KMoffett AAC Fanatic!

Dec 19, 2007
2,718
274
How do you make a 12 hour timer out of a CD4047 monostable/astable multivibrator and a relay?

Ken

5. ### mik3 Senior Member

Feb 4, 2008
4,846
69
You set it as a monostable and determine the resistor-capacitor value to make a delay of 12 hours. Use a relay on its output to drive your load. Read the datasheet for how you connect it.

6. ### eblc1388 AAC Fanatic!

Nov 28, 2008
1,543
104
This is interesting. Let me try to calculate the values needed.

12 Hours is 12x60x60 seconds = 43200 seconds.

CD4047 datasheet gives typical monostable timing to be 2.48*R*C, with limit on R to be 1MΩ max., and C to be non-polar for good timing repeatability.

One only requires a non-polar cap of 17,420uF and bingo, a 12-hr timer.

7. ### KMoffett AAC Fanatic!

Dec 19, 2007
2,718
274
Thanks for the calculation L.Chung. I thought it would be in that range.
I think the CD4060 and a 4011 would allow more reasonable component values.

Ken

Change: To hold the relay ON for 12 hours and then go Off, move R5 from pin3-U2 to pin4-U2. And use the Com/NC contacts on the relay.

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Last edited: Jan 8, 2009