High speed 2 input AND circuit

[mikesmixes]

I need o build a circuit that reads two input signals (high=24V, low=0V),
and when both are high then switches the N/C on a relay and cuts out the machine. I need to use a solid state relay cos of the speed.

Any ideas? I used a AND ic before and a normal relay, but it was to slow.

 

[Styx]

For speed I would say make a descrite ECL but you have a high voltage level

use "micky-mouse logic"

Make a diode AND gate and use an output MOS for driving power. Use fast diode (schokky) and fast FET should do it

 

[k7elp60]

How fast do you want to switch of the machine? Most mechanical relays operate in about 15 milliseconds.
A dual comparator with a low voltage reference and voltage dividers on the two inputs would possibly work for your and gate.

 

[Roff]

What is the load? How much current are you switching? Is the load voltage also 24 volts?

 

[mikesmixes]

There are two inputs. one being a lazer pickup and the other a proximity switch. Both switch +24v. When both of these are high (+24V), i need the machine to stop. The control voltage for the machine is only 24v. I am using the N/C on a relay for this.

My first circuit consisted of: 2 inputs thru a voltage divider into a dual opto. The opto switched +12V from a voltage regulator and the 2 signals went into an AND gate. The output then switched the coil of the relay.

It works fine but at a low frequency and on the bench. Now I have to do some signal conditioning after the opto and before the AND gate. The lazer pickup signal is very distorted and not smooth so the AND gate doesnt pick it up. I will be putting a inverting shmitt trigger and then a normal inverter after the opto, and this will go into the AND.

Will let you know what happens.

 

[panic mode]

First a warnning:
I hope you are not trying to make your own safety circuit. It is ok if you do it for yourself but don't think of trying to use it on machine that will
be used by others. There are regulations on machine safeguarding so
you better be aware of it. Most "user level" safety has to be done using
hardwiring and safety relays (hence slow response in the 20ms range).

What is the make and model of the proximity switch you use?
Most of them are quite slow (ca 1ms or so, smaller ones usually
have faster switching times). An AND gate in TTL or CMOS will
easily outrun this by long shot. Relay is mechanical device and hence
10-15ms switching times.
You can get faster response using solid state relays but using
transistors could get way faster response.

 

[mikesmixes]

firstly i am not trying to make a safety circuit or something.
This circuit is going is used on a textile weaving machine. The lazer shines thru the yarn and detects when a yarn is broken. The proxy senses when the "pick"has returned back to its normal position.

So all this circuit is doing is "if" the yarn has broken AND the "pick" is in its normal position then the machine must stop otherwise there will be gaps in the material.

I will try the shmitt tomorow then post again.