7414 Schmitt Trigger Not Working At All

Hi,
I am driving a 7414 Schmitt Trigger from a PS2505 optotriac I have a 120VAC switch coming in to the PS2505, the output of the PS2505 going to a 7414 Schmitt Trigger and the Schmitt output going to an Arduino input pin.

I have triple checked that I have 5V at pin 14 and GND and pin 7 of the Schmitt. I ave also swapped out the 7414 for another one with no change

I have tried all of its gates:
Pin 1 IN : Pin 2 OUT
Pin 3 IN : Pin 4 OUT
Pin 5 IN : Pin 6 OUT
and so on…

When I tie any input LOW (to GND) its corresponding output goes HIGH and vice versa.

But when I drive it from the PS2505 via a 120VAC switch it doesn’t work

I have attached the relavent datasheets and a schematic of the section

74HC7014_Hex_NonInvertingSchmittTrigger.pdf (137 KB)

PS2505_datasheet.pdf (117 KB)

I'm not sure your hookup to the PS2505 will work. Try grounding the emitter, put a 4.7K resistor pullup on the collector and feed the 7414 from the collector.

For a regular TTL chip you need ~470 to GND on an input to guarantee a low. On LS TTL 1.2K will work.

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Thank you both for your replies.

Based on what you have both suggested I have drawn a new schematic (see Schematic#1 attached), however if I am correct there is a voltage divider between the output of the PS2505 and the input of the 7414.

If that is the case, should I do something like schematic#2 attached ?

Or should I do something else entirely?

Number 2 has the proper placement for the 470. Number 1 should work also, but you would not need the 470 in that example.

Number 2 |500x326

Number 1 |500x178

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Number 1 is also an inverting output in relation to the input, if that matters any.

You shouldn't even remotely think of using TTL these days, A 74HC14 (CMOS) schmitt-trigger will have the voltage thresholds much more sensibly nearer to mid-rail, and present no load on the input circuit.

For a regular TTL chip you need ~470 to GND on an input to guarantee a low. On LS TTL 1.2K will work.

You shouldn't even remotely think of using TTL these days, A 74HC14 (CMOS) schmitt-trigger will have the voltage thresholds much more sensibly nearer to mid-rail, and present no load on the input circuit.

The IC that Rosscopicoptrain provided a link to in his original post was in fact a CMOS version, 74HC7014. Referring to it by the generic name 7414 might have caused some confusion.

It looks to me as though the circuit in question is meant to be a zero voltage detector for use in a phase controlled power controller.

If that is the case, (and as far as I can see that has not been mentioned yet in this topic), then I think that there is a major problem with the circuit.

Using a capacitor and resistor to drop the voltage to the LEDs is going to give a phase change to the current through the LEDs. (I know you have done it to reduce power dissipation in the circuit as a whole).

Using an on-line calculator, it looks to me as though there is a 72° phase difference between the current through the LEDs and the main current. As far as I can see, there will always be this 72° offset.

The calculator also suggested an impedance of around 8.3kΩ, I'll discus the implications of this in my next post.

JohnLincoln:
The IC that Rosscopicoptrain provided a link to in his original post was in fact a CMOS version, 74HC7014.
Referring to it by the generic name 7414 might have caused some confusion.

Yes, wrong part numbers always adds to confusion I find.

JohnLincoln: If that is the case, (and as far as I can see that has not been mentioned yet in this topic), then I think that there is a major problem with the circuit.

Using a capacitor and resistor to drop the voltage to the LEDs is going to give a phase change to the current through the LEDs. (I know you have done it to reduce power dissipation in the circuit as a whole).

That part is a circuit I found as I am not entirely familiar with using reactance when designing circuits.

Another question, I am trying to do hardware debouncing between the PS2505 and the 74HC7414. However all of the debouncing circuits I have found so far have a 10K pullup and a 0.1uF to ground. Since I am using a 470 to ground, should I be putting the capacitor between the PS2505 output and VCC ?