On Thu, 20 Nov 2008 13:27:10 -0800 (PST), Graham
put finger to keyboard and composed:

Discreet component monostabe constructed from a ‘TTL’ OR gate ,
resistor and capacitor , Im trying to determine what the pulse width
should be from the timing components, but I don’t see any info on this
configuration. I need to certify the cct is in specification .. Its
fed with a 0 5v step

From a common start point, one line direct to gate (1) input, from
the same common start point, a 220 ohm resistor connected to the
second gate (2) input.

From the junction of the 220 ohm resistor and the second gate input is
connected a 0.0022nF capacitor to ground.

The resistor and capacitor form a cr timing network.

Operation appears to be , the initial 'ON' voltage (5v) is applied
direct to gate (1) , the capacitor to ground delays the voltage
applied to gate (2) charging via the 220 resistor.

When the two inputs are at logic ‘1’ then the ‘OR’ Fnction changes the
gate state .

Im seeing a 650nS pulse , has anyone a formula or a chart that gives
a guide to pulse duration ?

An OR gate will have an output of 1 whenever input #1 is a logic 1,
regardless of what happens on input #2. OTOH, an XOR gate (eg 74LS86)
will behave as you describe.

A Motorola SN74LS86 has a guaranteed minimum input high voltage of
2.0V. This means that the output is guaranteed to change state when
the capacitor charges up to 2.0V. However, it may also change state at
1.5V, so trying to compute a pulse width would be difficult. All you
can calculate with any reasonable certainty is the *maximum* pulse
width, although this figure would also be affected by the current draw
into or out of the gate.

If we calculate the RC time constant, then ...

R x C = 220 x 0.0022 nF
= 0.484 ns

.... which is 1000x lower than you have measured.

Are you sure you didn't mean C = 0.0022uF ? This would result in an RC
value of 484ns.

The formula for the charging voltage, V, would be ...

-t/RC
V = Vo (1 - e )

.... where Vo = 5V.

The trigger point would be when V = 2.0V

This reduces to ...

t = -RC ln(1 - V/Vo)
= 247ns

- Franc Zabkar
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