The hope was to get extended charge time out of an LM555.
LM555 normal operation.
On startup the capacitor is discharged “trigger level” is met. Discharge is released and the capacitor starts to charge. Eventually the capacitor charges up to the Threshold level (2/3 of VCC, or about 1 time constant). When Threshold is met Discharge goes low and the capacitor starts to discharge. When the charge gets down to about 1/3 of VCC the Trigger point is hit and Discharge is released again, starting the cycle over again.
Circuit A is the standard oscillator design. R (10K) and C (100 µF) has a time constant of about 1 second. In this standard we should, and do, get a time cycle of about 3.5 seconds. About 1 second charge time and about 2 seconds discharge time.
Circuit B. The hope was to get about two RC time cycles on the charge by feeding back only a portion of the capacitor charge back to the Threshold pin. To my surprise I get a cycle of about 10 Hz.
Yes, I know I am losing the discharge RC time. I was still expecting a cycle of about 2 Hz.
What went wrong?
LM555 normal operation.
On startup the capacitor is discharged “trigger level” is met. Discharge is released and the capacitor starts to charge. Eventually the capacitor charges up to the Threshold level (2/3 of VCC, or about 1 time constant). When Threshold is met Discharge goes low and the capacitor starts to discharge. When the charge gets down to about 1/3 of VCC the Trigger point is hit and Discharge is released again, starting the cycle over again.
Circuit A is the standard oscillator design. R (10K) and C (100 µF) has a time constant of about 1 second. In this standard we should, and do, get a time cycle of about 3.5 seconds. About 1 second charge time and about 2 seconds discharge time.
Circuit B. The hope was to get about two RC time cycles on the charge by feeding back only a portion of the capacitor charge back to the Threshold pin. To my surprise I get a cycle of about 10 Hz.
Yes, I know I am losing the discharge RC time. I was still expecting a cycle of about 2 Hz.
What went wrong?
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