Need help designing a simple project.

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bena
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Need help designing a simple project.

Post by bena » Mon Oct 06, 2008 4:37 pm

Hi all,

I need a circuit that does the following. I will have a light sensor attached to an LED. When the LED turns on, I want this circuit to momentarily close a circuit ( act as a switch ) for another circuit board. Then when the LED turns off, I want to either close the same circuit as before or close a different circuit, either way would work. I want to avoid having the circuit stay closed as long as the LED is on as that will adversely effect the battery life of the 2nd circuit.

I would like this to operate on 2.5-3v

Thanks for the help

Ben

Bigglez
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Re: Need help designing a simple project.

Post by Bigglez » Mon Oct 06, 2008 6:33 pm

bena wrote:I will have a light sensor attached to an LED. When the LED turns on, I want this circuit to momentarily close a circuit ( act as a switch ) for another circuit board. Then when the LED turns off, I want to either close the same circuit as before or close a different circuit, either way would work. I want to avoid having the circuit stay closed as long as the LED is on as that will adversely effect the battery life of the 2nd circuit.
I would like this to operate on 2.5-3v
I'm sure you'll get lots of suggestions from this forum,
before I throw my hat in the ring I have a couple of
questions:

(1) The LED that triggers your circuit is presumed to be
existing and you can't touch it (make electrical connection),
correct?

(2) The light sensor is new, and must respond to the LED,
correct? What colour is the LED?

(3) The sensor is powered by battery (2.5 to 3V)?

(4) The two slave circuits are operated by the sensor?

(5) The slave circuits are triggered by the change
in the LED (off to on and on to off), but ignore the
steady light?

You have not given us the electrical demand of the
slave circuits. Are they also low voltage or perhaps
AC powered? What "volts' and 'amps' do they need?

When you say "momentarily" do you mean less than one
second or more than one second? Do you know how long?
Is this critical?

This is not the Spanish Inquisition, just trying to define
what you need and already know.

Is this your project or a school assignment?

bena
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Post by bena » Mon Oct 06, 2008 7:35 pm

1. Can not touch the LED
2. Red LED
3. The circuit with the sensor will run off 2 AA batteries
4. Yes
5. I would like it to be a change of state that causes the slave circuit to close the circuit of the main board. Yes to ignore the steady light, or else it will run

Less than a sec is fine.

Thanks

Bigglez
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Post by Bigglez » Mon Oct 06, 2008 7:47 pm

bena wrote:1. Can not touch the LED
2. Red LED
3. The circuit with the sensor will run off 2 AA batteries
4. Yes
5. I would like it to be a change of state that causes the slave circuit to close the circuit of the main board. Yes to ignore the steady light, or else it will run
You have not given us the electrical demand of the
slave circuits. Are they also low voltage or perhaps
AC powered?

1. What "volts' and 'amps' do they (the two slaves) need?

2. Is this your project or a school assignment?

bena
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Joined: Mon Oct 06, 2008 4:27 pm
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Post by bena » Mon Oct 06, 2008 8:08 pm

everything is running off 2 AA batteries.

Basically I need this circuit to drive an IR remote, pressing the remotes buttons based on the condition of the LED.

This is my project.

Bigglez
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Post by Bigglez » Mon Oct 06, 2008 11:54 pm

bena wrote:everything is running off 2 AA batteries.
Basically I need this circuit to drive an IR remote, pressing the remotes buttons based on the condition of the LED.
This is my project.
When you say "pressing the remote's
buttons" do you mean phyically, or do you
mean by electrical equivalent of the existing button
circuits?

sghioto
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Post by sghioto » Tue Oct 07, 2008 10:42 am

bena,
Here's a circuit that might work for your application. Using a HTC550 monostable with a pulse width of .1 second duration. Circuit works with the components values shown in the schematic using an ordinary cadmium photocell as the sensor. R1 may need to be adjusted depending on the type of light sensor used. The HTC550 responds to a positive or negative trigger pulse depending on the state of pin #7, the trigger slope. In this circuit the trigger slope alternates from high to low allowing the sensor to trigger when it's on and off. Output circuit can be modified if needed. One thing to take into consideration is the circuit will pulse once when power is first applied.
http://www.hightechips.com/Download/htc550.pdf

Cheers,
Steve G.

Image

Bigglez
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Post by Bigglez » Tue Oct 07, 2008 11:10 pm

Was reading this datasheet, but I don't know what this means:

Single chip solution for most pulse starching applications

I used to get my work shirts starched...

bena
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Post by bena » Wed Oct 08, 2008 7:47 am

Thanks so much for the design Steve.

I have one other hopefully simple circuit to design.

I need a circuit that takes TTL output and turns an LED on and off. My understanding is that the TTL will send 1 pulse for ON and 2 pulses for off. I am trying to get the complete specs on the TTL output.

Thanks again.

Ben

Bigglez
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Post by Bigglez » Wed Oct 08, 2008 3:56 pm

bena wrote: I need a circuit that takes TTL output and turns an LED on and off. My understanding is that the TTL will send 1 pulse for ON and 2 pulses for off. I am trying to get the complete specs on the TTL output.
TTL (Transistor-Transistor-Logic) is a spec for signals
that were originally used with bipolar logic function ICs.
It specifies logic thresholds; loading (Fan-In); and
driving (Fan-out); and edge speeds (slew-rates).

Over time the semiconductor performance improved
and the industry switched to CMOS for a variety of
advantages. The logic levels in newer families are
said to be TTL compatible (some more than others)
to allow the different types to mix reliably.

You need to know more about the signal you have
in order to decode it for your project.

Most likely you can differentiate between the one-pulse
and two-pulse signals with a monostable multivibrator
(a flip-flop) and a latch (to remember the last state)
Either CMOS or TTL chips will drive a simple LED
directly.

This would be a trivial project for a small uC
(microcontroller), and would be a simple one 8-pin
chip solution. However, a uC runs firmware and requires
one time programming for a specific application.

bena
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Post by bena » Wed Oct 08, 2008 7:41 pm

Thanks Bigglez,

I just found out that the output is actually just a High Low TTL, so I can just use a transistor to interface the way I need.

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