| Electronics > Beginners |
| 74LS14 testing circuit pls |
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| Chriss:
I think I have to put another value for the R3 but I can't figure out what value could it be. |
| PA0PBZ:
--- Quote from: Chriss on April 18, 2018, 02:20:08 pm ---If I remove P832 the 7414 doesn't go high, but If I short the 7414 input to GND then the signal goes high. I also tried with a simple transistor BC337 as a switch wired up to the input of 7414 and worked fine. --- End quote --- Did you try my other suggestion? |
| Chriss:
--- Quote ---Did you try to just short the 74ls14 input to ground with the P832, without using any resistors? --- End quote --- Yes, I tried, nothing happens. Signal on my scope still high. But here is something I found in the datasheet of the P832: I set the R3 to 1K and connected the Colector to +5v. With this setup the 7414 works great on the scope. I'm a bit afraid does I do something wrong? Or can I use the above schematic to make my project working? I have to setup another 3 photo interrupter to the 7414. What do you think? |
| Ian.M:
You cant pull a TTL input low reliably with a resistor over a few hundred ohms so putting the phototransistor between In and Gnd is essential as it probably cant handle driving a 470R resistor with its emitter. Once you've moved the phototransistor, I'd put a pullup resistor on the input as well if the phototransistor has enough drive to handle the extra current and still reach a valid logic '0' level with plenty of margin. CMOS 74HC/HCT here would be a lot easier - it removes most of the constraints on resistor values etc. caused by the input current that has to be sunk to pull the input low and lets you use the Toshiba datasheet circuit with a much higher resistor value. If you don't want a low going pulse when the light path is interrupted, use another gate of the 74xx14 to invert the output. If you *MUST* have more than 3 inputs on the same chip and *MUST* use a 74LS14, you'll need to add extra transistors to turn the phototransistors into Darlingtons so they can handle the approx 10mA to drive high with a 470R pullup. Any jellybean small NPN will do, but put 1K B-E to sharpen up the turnoff. Personally, I wouldn't bother, unless its driving a lot of other gates or is in a high risk environment for ESD I'd just drop in a 74HCT14. |
| StillTrying:
How do you expect the opto to pull the signal high with R1 there. This should work with any version of '14 inverters, - it's as in Ian's 1st paragraph. |
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