Mail's here Project(problems encountered)

Thread Starter

iONic

Joined Nov 16, 2007
1,650
Having a bit of trouble with getting this circuit to operate properly.

How it should work:
The A and B sections of the 4011 quad NAND gate U1 form a cross-coupled latch. If the photodetector is dark (i.e., high resistance) and the pushbutton is closed momentarily, the output of (pin 3) is forced low. It remains in this state when the button is released.

When the mail box is opened, the CdS sensor is illuminated and its resistance falls, pulling pin 1 low. The output of U1A then goes high and remains that way even when the CdS sensor returns to its dark, high-resistance state. Pushing the button (SW1) flips the latch back to its original state, with U1A low.

The output of U1A goes to a gated oscillator circuit made up of U1C and U1D. This oscillator is stopped when pin 8 (driven by output U1A) is low. U1D's pin 11 is also low, turning the LED driver transistor off. When U1A switches high, the gated oscillator runs with a period of about 2 seconds, causing the LED to cycle - 1 second on and 1 second off.

How it is working for me:
When I apply power, the LED turns on and blinks at the rate of about 1sec on/1sec off. It does this at very low light and at very high light and even when I remove the LDR & 0.1uF cap. The reset button fails to "reset."
Given these conditions, it appears that the oscillator section is working fine.

What's wrong? Wiring? ....Components?
MailsHereCircuit.JPG

The A and B sections of the 4011 quad NAND gate U1 form a cross-coupled latch. If the photodetector is dark (i.e., high resistance) and the pushbutton is closed momentarily, the output of (pin 3) is forced low. It remains in this state when the button is released.

When the mail box is opened, the CdS sensor is illuminated and its resistance falls, pulling pin 1 low. The output of U1A then goes high and remains that way even when the CdS sensor returns to its dark, high-resistance state. Pushing the button (SW1) flips the latch back to its original state, with U1A low.

The output of U1A goes to a gated oscillator circuit made up of U1C and U1D. This oscillator is stopped when pin 8 (driven by output U1A) is low. U1D's pin 11 is also low, turning the LED driver transistor off. When U1A switches high, the gated oscillator runs with a period of about 2 seconds, causing the LED to cycle - 1 second on and 1 second off.
 

Thread Starter

iONic

Joined Nov 16, 2007
1,650
HA!
Thought I had checked this out. (resistor for LDR). was at 22k, but for my light conditions and LDR it seems that 1-2K is about right. Will be using a 5K pot to replace 22K(R4) for an adjustable sensitivity as I will be adding 100+ feet of wiring to reach my mailbox and will adjust under live conditions.

All is well. Now I need to build it for real!

Yes, I would consider using a wireless component, but the harsh winters in New England tend to kill batteries fast.
 
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