# 74HC221 negative edge trigging

How to use 74HC221 for output variable pulse width on negative edge.

I tried connect B and MR pins to HIGH and A pin to input as tells in datasheet. But it give huge pulse like 1 second.

Probably I must use RX and CX pins but don't know how to use and datasheet doesn't tell.

The pulse that i want, must be between 0-10 ms.

I add R4, R5 and C3 change the values of R4 and C3 multiple times but it didn't change the output.

Then realized this:
"Pin 'CX' is not modelled.
Pin 'RX/CX' is not modelled."
in simulation log. Could this be the problem?

• Perhaps you could add a bit more context to improve your question. What have you tried? What didn’t work? What are you trying to achieve? Mar 18 at 14:08
• Add a resistor from the base of Q1 to GND for a more lean and snappy turn-off, Also, the regulator needs a capacitor directly at its input, per the datasheet. Mar 18 at 14:34
• The chart in the lower image has no useful information, just large color blobs. What are you trying to show? Mar 18 at 14:36
• @AnalogKid The blue is output from Q. Yellow is Transformer and red transistor their period is 20ms so the problem and want to show the blue is 1s Mar 18 at 15:07
• @akiftokel - Also be aware of the limits of RX, as shown on page 1: "The minimum value of external resistance, RX, is typically 500Ω. The minimum value of external capacitance, CX, is 0pF. The calculation for the pulse width is tW = 0.7 RXCX at VCC = 4.5V."
– Nedd
Mar 18 at 15:29

The equation for calculating the values is on page 1 of the datasheet. Also, figure 8 shows the relationships among R, C, and time.

For your desired time period, you can pick approximate R and C values from the chart, and then fine-tune them for commercially available values with the equation.

The TI 74HC221 data sheet clearly shows (on page 2) where the RX,CX components are connected.
For proper operation follow the truth table (same page), be sure you satisfy all input pins.

Input $$\B\$$ operates with a rising edge, while $$\\overline{A}\$$ operates with a falling edge. ( Input $$\\overline{R}\$$ must be held high).

Below is the page 2 that I see. If this is not what you see your PDF reader App may be outdated.