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Simple delay for Sensor

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TimK

New Member
Hi
I have a proximity sensor thats output is connected to a PLC in a piece of equipment. Its function is to shut the machine down when metal is detected. What happens is when metal is detected by the sensor 24Vdc is supplied to a pin on the PCL, the PLC then instantly shuts the machine down.

What I need to do is intercept the 24Vdc ,wait a few seconds and then send it to the PCL. If the 24Vdc is lost during the wait time then start the wait time over.

Anyone have a circuit for this?

Thanks
 

MikeMl

Well-Known Member
Most Helpful Member
What power supply voltage is available?

Are you adverse to using a small relay?
 

TimK

New Member
24Vdc or 120Vac are the only voltages available except for the 480 3 phase. I would prefer to use an electronic device as apposed to a relay. Since the sensor will be cycling continuously. In other words metal objects pass the sensor all the time. I just want the signal sent to the PLC if the sensor detects metal for several seconds.

HTH
Thanks
 

MikeMl

Well-Known Member
Most Helpful Member
Custom designed just for you
 

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TimK

New Member
Mike, This looks great but I have questions,
Could you provide more details as to the wattage of the resistors and the types of caps in the circuit.

If I am reading the graph correctly at the bottom the delay looks to be about 2 seconds which is exactly what I need. Would it be difficult to add a pot to vary the delay a few seconds +-. (not necessary just an option)

What does the RC at C1 represent. Is it just a reference point for the current input voltage. I am just now at 50 and trying to learn something new. So some of these questions may sound dumb.

Thanks
TimK
 

odps

New Member
Plc

TimK

You should be able to make some very slight program changes in the PLC by adding a timer and let the PLC do the delay for you???

What brand of PLC is being used?
 

MikeMl

Well-Known Member
Most Helpful Member
Mike, This looks great but I have questions,
Could you provide more details as to the wattage of the resistors and the types of caps in the circuit.

If I am reading the graph correctly at the bottom the delay looks to be about 2 seconds which is exactly what I need. Would it be difficult to add a pot to vary the delay a few seconds +-. (not necessary just an option)

What does the RC at C1 represent. Is it just a reference point for the current input voltage. I am just now at 50 and trying to learn something new. So some of these questions may sound dumb.

Thanks
TimK

Revised schematic attached. The "RC" is just a node label so that I could plot that node. I added a 1megΩ Pot to vary the delay. The caps over 1uF are 15V electrolytics. The 100nF is just a bypass, non polarized. All resistors are 1/4W metal film. The Zener is 1/2W 12V. The 555 is available from lots of sources.

Except for the Zener and the Pot, you might even be able to get the parts at RatShack :D
 

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Last edited:

TimK

New Member
Thanks Mike
This is great.

And Hey! Be Nice, I used to manage a "RatShack" LOL But that was back before they became a retail outlet for other brands.

I am going to try my hand at designing a PCB. When I get it done I will post it here. Maybe you could look at it to make sure I dont shut down the power to the central US or something.

Thanks Again
 

MikeMl

Well-Known Member
Most Helpful Member
Here is a description of the circuit I posted:

The 555 Timer is configured as a retriggerable one-shot multivibrator. As long as the Sensor input is low, diode D1 conducts, discharging C1 to near ground. Because the voltage at the 555's Trig pin is below 4V (1/3 of 12V), this puts the 555 into the "triggered" state, where its output pin 3 is high, which causes the collector of Q1 (Output) to be low because Q1 is turned on.

When Sensor goes high (near 24V), D1 is reverse-biased, allowing R1 to charge C1 toward 12V (from the Zener). It takes R1 a few seconds to charge C1 to 8V (2/3 of 12V). This causes the 555's Thresh pin to put the 555 into the "reset" state, where its output pin 3 is low, causing Q1 to turn off, allowing R4 to pull the Output all the way to 24V. If the Sensor input goes low before C1 reaches 8V, C1 is rapidly discharged via R2 and D1, preventing it from timing out. Only after Sensor goes high and stays high for longer than the timing period will the Output go high...

24V exceeds the maximum allowed operating voltage, so I used a Zener regulator to operate the 555 from 12V. Since the Sensor input swing is from 0 to 24V, I returned the pull-up resistor on the output to 24V. C2 and C3 are bypass caps...
 
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