Hi all,
I'm designing a test circuit that sequentially checks 12 TC junctions for correct polarity. I'm using a 555 oscillator and a pair of cascaded 4017's to sequence the tester. The tester looks at just one junction at a time.
BTW, thanks to all who helped me get two 4017's to cascade and reset correctly. That part of this project is working great.
As each TC junction is heated, the mV output is sent to an amplifier and then to a LM339 quad comparator. When that amplified voltage exceeds the LM339's reference voltage (2V), the output goes to a 4081 AND gate along with the output from the 4017 (this verifies the specific junction under test). I can use the output (5VDC) from the LM339 to light a LED. So far, so good.
Here's where I'm stuck: As each junction test finishes, I need to keep the LED lit until all the junctions have been tested so that the assembler can see that the test sequence is complete.
Obviously, what I need to have is an IC that, when it receives a high output signal from the comparator, switches on the LED (either directly or via a transistor) and holds it in that state until the operator resets the tester. I've done a fair bit of research for the device I need, and it looks like a 4042 quad latch may be the answer. I've tried to find a circuit example that uses the 4042 to perform the LED latching function, but haven't found anything yet.
I don't have a schematic at this time (still learning to use ExpressSCH). I've searched the forums here for a thread that addresses this specific issue but haven't found one as yet. I really just need to know what to do once I have the 5VDC output from the LM339. Can anyone point me in the right direction? As always, all help is much appreciated!
I'm designing a test circuit that sequentially checks 12 TC junctions for correct polarity. I'm using a 555 oscillator and a pair of cascaded 4017's to sequence the tester. The tester looks at just one junction at a time.
BTW, thanks to all who helped me get two 4017's to cascade and reset correctly. That part of this project is working great.
As each TC junction is heated, the mV output is sent to an amplifier and then to a LM339 quad comparator. When that amplified voltage exceeds the LM339's reference voltage (2V), the output goes to a 4081 AND gate along with the output from the 4017 (this verifies the specific junction under test). I can use the output (5VDC) from the LM339 to light a LED. So far, so good.
Here's where I'm stuck: As each junction test finishes, I need to keep the LED lit until all the junctions have been tested so that the assembler can see that the test sequence is complete.
Obviously, what I need to have is an IC that, when it receives a high output signal from the comparator, switches on the LED (either directly or via a transistor) and holds it in that state until the operator resets the tester. I've done a fair bit of research for the device I need, and it looks like a 4042 quad latch may be the answer. I've tried to find a circuit example that uses the 4042 to perform the LED latching function, but haven't found anything yet.
I don't have a schematic at this time (still learning to use ExpressSCH). I've searched the forums here for a thread that addresses this specific issue but haven't found one as yet. I really just need to know what to do once I have the 5VDC output from the LM339. Can anyone point me in the right direction? As always, all help is much appreciated!