Below are schematic and board images for my circuit. Its job is to generate a linearly increasing signal at the output of around ~229 mV per min rate of rise. (The output signal is used to calibrate another instrument).
LED1 sets the control voltage at around 2V, so the voltage across C1 should be a sawtooth that cycles between 1V and 2V, with a period of 254s.
R8, C3, and Q2 make up a timing circuit to charge C1 quickly upon startup.
R1, Q1, and D1 provide a constant charging current of 12.6uA into C1.
The problem is this: a couple of seconds after startup, C1 passes the threshold voltage of 2V. At this point, the NE555 discharges C1. The NE555 output drops low. So far, so good.
But....the NE555 reset pin is also dropping low at this point. So the NE555 output stays low, the discharge continues to sink current, and C1 does not begin to charge. Expected behavior is for the NE555 output to go high when C1 has dropped to 1V, and begin the slow charging cycle.
I've modeled this extensively and have not yet found the flaw in the design. I have also built and used a similar circuit successfully (the differences being that this new circuit's sawtooth is 10X steeper, plus the addition of the R8/C3/Q2 quick charge circuit at startup).
What really has me scratching my head is that the 555's reset pin is going low despite it being tied to V+.
Thanks for any suggestions.
Jim
LED1 sets the control voltage at around 2V, so the voltage across C1 should be a sawtooth that cycles between 1V and 2V, with a period of 254s.
R8, C3, and Q2 make up a timing circuit to charge C1 quickly upon startup.
R1, Q1, and D1 provide a constant charging current of 12.6uA into C1.
The problem is this: a couple of seconds after startup, C1 passes the threshold voltage of 2V. At this point, the NE555 discharges C1. The NE555 output drops low. So far, so good.
But....the NE555 reset pin is also dropping low at this point. So the NE555 output stays low, the discharge continues to sink current, and C1 does not begin to charge. Expected behavior is for the NE555 output to go high when C1 has dropped to 1V, and begin the slow charging cycle.
I've modeled this extensively and have not yet found the flaw in the design. I have also built and used a similar circuit successfully (the differences being that this new circuit's sawtooth is 10X steeper, plus the addition of the R8/C3/Q2 quick charge circuit at startup).
What really has me scratching my head is that the 555's reset pin is going low despite it being tied to V+.
Thanks for any suggestions.
Jim
Attachments
-
71.6 KB Views: 42
-
134.9 KB Views: 25