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need TLC551 help

Looking at the data sheet, I note that the device is characterised at

2V, 5V and 15V. In particular, if you look at the t(plx) graphs on page 9, you will notice the times going skyward at around ~3V increasing exponentially approaching 2V. Tellingly, the graph stops at 2V.

As the spec information doesn't tell me what happens at 1V (and for CMOS we can guarantee it will be **slow!!**), I would say it should not be *used* below 2V.

Do you have access to the TI xPIC (Product information centre)? I use them regularly when the datasheet doesn't tell me what I need to know. (EPIC for Europe). Alternatively, you might want to hunt up your local TI rep FAE.

Cheers

PeteS

Reply to
PeteS
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I wonder if he could sidestep that problem by using the output to drive the oscillator, rather than relying on the discharge pin? If he connects a 1MEG resistor to the output, to a 15uF cap to ground, and drives the trigger and threshold pins using the junction of these two, he can get timing that does not rely on the low impedance of the discharge pin.

However, if the discharge pin is high impedance, it may not be able to pull down the shutter pin on his camera anyway.

The OP could probably use a coin or watch cell to add some voltage if he is unable to get the timer working otherwise.

--
Regards,
  Bob Monsen

If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
                                  Thomas Henry Huxley, 1877
Reply to
Bob Monsen

I need some help in using the TLC551 timer chip in a digital camera application.

formatting link

I need to trigger a tiny digital camera for time lapse photos. The camera uses one AAA cell so I would like to use the TLC551 if I can, which can work down to 1 volt, so I don't need to supply the timer with a separate power supply.

All the timer needs to do is short the "shutter release" switch contact to ground every 5 to 10 seconds. I came upon a design using a 7555 at

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which does this using an unusual method, but unfortunately, the 7555 only works down to 2 volts on a good day.

I am unsure about the passive components needed to time the TLC551 to do this, or even if it can be done. The data sheet would indicate that the math is the same as the TLC555 or 7555, but this does not seem to be the case when I breadboard the TLC551.....maybe due to the 1 to 1.5 operating voltage or whatever that I am trying.

Can someone help me with this issue?

Thanks Jim

Reply to
Jim

You might be able to use a couple of 3V coin cells and your a 551 into an optoisolator.

Ed

Reply to
ehsjr

It looks like the camera circuit has a step-up regulator so all this low voltage may not be necessary if you can tap into that. However, if you insist on running this from the battery, then the diode drop due to D1 in the link circuit is way too large of a fraction of Vbatt for reliable operation. You can remove the diode, add 3x resistors and 2x transistors to make this thing work , like so:

View in a fixed-width font such as Courier.

Vbatt | +-----------------+----+-----------------+ | | | | | | | | | +----------|----|-------+ | | | | | | | | | | | | | | | +-------------+ | | | [680K] | V+ RST | | e | | | | | |/ | +-------|DIS OUT|-+-[1K]--| PNP | | | | |\\ | | | | c === | | | | 0.1U [68K] +--|THRESH | | +---> to shutter | | | | | [1K] | | +----+--|TRIG | | | | | | | | | | === | CON| | c | | 10u | | | |/ | |TANT | GND | +---| NPN | | +----------TLC551 | |\\ | | | | e | | | [100K] | | | | | | +------+--------------+------------------+-----+ | --- ///

Reply to
Fred Bloggs

There is no need to tie the 680K pull-up to the OUT pin because the current is so small. You can tie it directly to Vbatt:

View in a fixed-width font such as Courier.

Vbatt | +------+----------+----+-----------------+ | | | | | | | | | | | | | | | | | | | | | | | | | | | +-------------+ | | [680K] | V+ RST | e | | | | |/ | +-------|DIS OUT|---[1K]--| PNP | | | | |\\ | | | | c === | | | | 0.1U [68K] +--|THRESH | | +---> to shutter | | | | | [1K] | | +----+--|TRIG | | | | | | | | | | === | CON| | c | | 10u | | | |/ | |TANT | GND | +---| NPN | | +----------TLC551 | |\\ | | | | e | | | [100K] | | | | | | +------+--------------+------------------+-----+ | --- ///

Reply to
Fred Bloggs

Seems as if the simplicity of design I was hoping for here is going to be lacking in trying to use the TLC551. Instead, I was allerted to check into the ZETEX ZSCT1555 timer IC which supposedly works down to .9 volts, and is advertized as a single cell (1.5v) 555 equivelant.

It is available in a DIP version, and am now wondering if it can do what I hoped to do described in my original post. It uses different timing math, but other than that looks a lot like the 7555 used in the "pin 7 output" design.

Any thoughts on using this alternative?

Jim

work

Reply to
Jim

Bad choice- bias currents too high for your timing duration, TRIG threshold as low as 0.15Vdd, THRESH at 0.8Vdd, and very weak DIS drive which comes out of saturation at 0.5mA at 1V or about 1/10 the rating of the ICM7555. And you have already been told what the story is on your "unusual" circuit with the diode.

[snip]

Thoughts? What are those?

Reply to
Fred Bloggs

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