eetech00, I am having problem on the Opto-Isolator input voltages. I am using ISQ5 from Isocom Dual Transistor Output Opto-Isolator and the forward voltage is 1.2 Volts. What I am getting right now is 1.016 Volts. Can you make a simulation if I used a 33K or lower resistance to get more than the required minimum voltage for the Opto_Isolator? Also that the reset signal requirements should be to switched off the negative power rail.

Hi

The datasheet doesn't have a graph showing the CTR. But at 1ma input current there will be 40% CT, so that's about 400uA for the output which should be plenty for the CMOS device inputs. I agree with Alex_T, the 36k or 33k should be fine, you don't need much current.

 

Here is a version modified with the reset control input from the ground rail.

 

That should be perfectly fine. The "1.2V" is the typical value for a forward current of 50mA (the absolute max), but you don't need that much current.

The data sheet stated a minimum of 1.2 Volts input. There is no change in the output. Maybe the component is defective.

 

The data sheet stated a minimum of 1.2 Volts input. There is no change in the output. Maybe the component is defective.

I read the datasheet and there is no chart showing forward voltage (Vf) vs forward current (If). This chart would show how much voltage is dropped across the diode with respect to current flowing thru it. If we had that chart then we could find Vf for 1ma forward current and calculate an appropriately sized resistor. The 1.2 volt value is the typical Vf at 50ma and is the test condition used to test the part....this is way more than the actual circuit needs. It is not the minimum Vf value.

I don't know how you are testing the part but we are using it as a switch/level shifter/isolater. The output will be either 12v or 0v depending on whether or not there is current flowing thru the opto's diode emitter. We really don't care that much about how much current is flowing in the output as long as it is minimal. The 4093's help clean up and condition the analog signal input thru its "Schmitt Trigger" behavior.

If you really want to set the resistor value accurately, then you need to get the Vf vs IF chart from the manufacturer. Sometimes they'll give it to you if you ask.

But really....I think you'll be just fine with the value used. Like I mentioned in an earlier post, I've already bread boarded and tested the circuit. Even with the PC817 opto's.

 

eetech00, I am very sorry I failed on this circuit. Maybe I was overwhelmed by many components but as you have said that I should have tested it by blocked. Actually I did, I was able to make it light and sometimes continuous but I could not get what I want. I went back to the simple design submitted by Alec_t and I have only one question to ask. Everything was perfectly fine and I get what I want to achieved. It latches when I pressed and release the latch button. The only problem is, if the alarm is there and pressed the acknowledge the LED stays lit but when the acknowledge was released the LED flashes. Alec_t and eetech00, I think I will be concentrating on this circuit because it is simple, easy to trouble shoot and less components and almost got what I want. Maybe need only a little bit of your help on the design of maybe a resistor or capacitor that maybe required to achieved the requirements. The SCR is fine because it latches when released and switches off when reset button is pressed. Please simulate the attached circuit and your further enhancement is greatly appreciated.

Moderators note: changed image from tumbnail to full image

 

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eetech00, I am very sorry I failed on this circuit. Maybe I was overwhelmed by many components but as you have said that I should have tested it by blocked. Actually I did, I was able to make it light and sometimes continuous but I could not get what I want. I went back to the simple design submitted by Alec_t and I have only one question to ask. Everything was perfectly fine and I get what I want to achieved. It latches when I pressed and release the latch button. The only problem is, if the alarm is there and pressed the acknowledge the LED stays lit but when the acknowledge was released the LED flashes. Alec_t and eetech00, I think I will be concentrating on this circuit because it is simple, easy to trouble shoot and less components and almost got what I want. Maybe need only a little bit of your help on the design of maybe a resistor or capacitor that maybe required to achieved the requirements. The SCR is fine because it latches when released and switches off when reset button is pressed. Please simulate the attached circuit and your further enhancement is greatly appreciated.

Moderators note: changed image from tumbnail to full image

That doesn't look like Alec_t's design. I guess your using a different circuit.

 

Yes it was, I just modified it using the SCR as the latch circuit.

 

The circuit you posted won't work because (a) there is insufficient gate current to trigger the SCR and (b) the continuous operation of the 555 means that the FET periodically shorts the SCR and so prevents it latching.
Here's a modification which should work, providing you can source a suitable SCR. Note that the EC103B is obsolete. Do you have an alternative?
In this mod, R2 has been reduced to 10k (you may need to reduce it further, depending on the SCR chosen) to increase the gate current, and D7/R10 (I deleted your original R10/R11) have been added to pull down the FET gate when the SCR fires. Depending on the SCR type, the LED current may need to be increased to exceed the SCR holding current.
Incidentally, the two LEDs I show are arbitrarily-selected white ones; you may prefer alternatives for an alarm indication. If you decided to use the FET to switch a much higher current load then my mod wouldn't really be suitable, because the FET gate voltage is reduced, courtesy of D7, below the ususal 10V or so needed to minimise the Rds(on) resistance.


Edit: It would be advisable to add a resistor (say ~68k) between the SCR gate and ground to reduce the likelihood of false triggering.

 

Here's an improved version, which better isolates the FET and SCR functions. When the SCR fires it switches off the supply to the 555. FET gate voltage is not now pulled down by a diode, so the FET could happily drive greater loads.

 

Tested the circuit and the voltage is now pulled down not to assert output from 555. The acknowledge now remains lit. The only thing that remains is the reset. I will have to removed the board from the socket to completely reset it. It is not resetting by simply reset button. Tried other values for the R10 (68K) like 36K but no response.

 

I may need to change the SCR to PD130AA or 2N5064. I think there is enough current feedback from D8 to keep the SCR from triggering.

 

It is not resetting by simply reset button.

Ah. Looks like LED current is getting back through the relay coil when the reset switch opens. Try using a Normally-Open type reset switch, repositioned so that it shorts the SCR when pressed, like this :

 

Most alarm circuits are of the "monitored" contacts. i.e. they require termination resistors. If the wire is broken you would generate a fault. Is that necessary in your circuit.

I designed a "relay logic" based shutdown system in the 80's when I didn;t know any better. The lines were short, on the order of 20 feet except two lines anyway. The Fire Alarm (FA) signal that generated a shutdown, I asked for an SPDT contact. If there was continuity, a LED would be illuminated, so you had a LED and a larger indicator in reverse states.

We had a habit of disabling the FAP signal 2x/year when the alarm in other parts of the building were being tested. The usefulness came because we knew when the FAP panel was reset.

 

KISS, my circuit is a normally open contact from a voltage source 48V to trigger the alarm, Lamp Test & Acknowledge. When acknowledge, it should remain lit until the reset is pressed and the alarm is gone. The reset signal is to disconnect the negative power rail to the circuit. But as Alec_t has pointed out that there is a feedback coming from the relay through the LED the fire the SCR. But I have disconnected the relay and still the same.

 

But I have disconnected the relay and still the same.

Then there must be some other current path. Have you tried using an N.O. switch, as I suggested, to short the SCR and so reset the circuit?

 

Then there must be some other current path. Have you tried using an N.O. switch, as I suggested, to short the SCR and so reset the circuit?

I have tried using the N.O. switch but no effect because it is already triggered. Maybe we should reset the supply from the positive side. I think that will be OK because when I removed the board from the system, it reset.

There is another solution which I want to test is putting the relay as the latch circuit.

 

I have tried using the N.O. switch but no effect because it is already triggered.

I don't see why you get no effect. Did you have the switch connected directly between the anode and cathode of the SCR? Once the Ack signal goes, shorting the SCR anode to cathode will turn it off.

 

I don't see why you get no effect. Did you have the switch connected directly between the anode and cathode of the SCR? Once the Ack signal goes, shorting the SCR anode to cathode will turn it off.

I tried to short it directly to ground means shorting anode to cathode yes the lights went off but if you release the short it lights back again.

 

I think I've got the final drawing and correct biasing of the SCR. Tested it and got satisfactory results.

 

Shouldn't you have a high value resistor from the cathode of D7 to ground to give he leakage currents a place to go?