NE555 timer sparks low-cost voltage-to-frequency converter
The combination of an NE555 timer and a Miller integrator yields a voltage-to-frequency converter that costs less than 50 cents.
Gyula Diószegi and János Nagy, Divelex Ltd, Budapest, Hungary; Edited by Charles H Small and Fran Granville -- EDN, February 21, 2008
In 1971, Signetics—later Philips—introduced the NE555 timer, and manufacturers are still producing more than 1 billion of them a year. By adding a few components to the NE555, you can build a simple voltage-to-frequency converter for less than 50 cents. The circuit contains a Miller integrator based on a TL071 along with an NE555 timer (Figure 1). The input voltage in this application ranges from 0 to –10V, yielding an output-frequency range of 0 to 1000 Hz. The current of C1 is the function of input voltage: IC=–VIN/(P1+R1).
As the voltage on C1 reaches two-thirds of VCC, the 555’s internal discharge transistor opens, and the voltage on C1 returns to one-third the voltage of VCC, the lower comparator threshold. At one-third this voltage, the discharge transistor switches off, and C1 again starts charging. The NE555’s output is high while C1 is charging and low while C1 is discharging. The product of the input voltage and the charging time of C1 is constant. Because the discharge time is shorter than the charging time, the following equation results for the output frequency: fOUT~VIN/(P1+R1)×C1×1/3VCC.
P1 calibrates the relationship between the output frequency and the input voltage. Because the discharge interval is approximately 30 µsec, the accuracy of the voltage-to-frequency conversion decreases as the frequency increases. If you assign 100 Hz to –1V and 1000 Hz to –10V, the error of conversion ranges from 0.3 to 3%. If you use P1 to calibrate the output frequency in the middle of the input-voltage range at –5V, then the conversion error will be less than 1.3% over the entire range. To improve performance, C1 should have a low dissipation factor. You can diminish temperature dependence if R1 has a low temperature coefficient and P1 is a multiturn, ceramic-metal potentiometer.
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Dear Yitzhak,
We had direct exchange of mail regarding this NE555 circuit also. Please built this circuit on a protoboard. It will work.
Janos Nagy - 2008-13-3 02:48:00 PDT -
No comments?, I'll have to assume that in the real circuit pin 7 of IC2 is connected to pin 1 of IC1 and a diode clamps pin 2 of IC1 to GND (with the cathode at pin 2) rather than the circuit shown.
Yitzhak Friedman - 2008-5-3 04:53:00 PST -
Are you sure that pin 1 (or 5) of IC1 is not connected to anything and there are no clamp diodes at pin 2 of IC1? This would make a difference.
Yitzhak Friedman - 2008-3-3 04:21:00 PST -
More then sixty circuits work with the same characteristics. The C1 is discharged by the Q1. The triangular waveform was measured on the input of NE555 and the spine on the output of NE555.
The wafeforms can be seen on the next page:
www.geocities.com/jsnagy10/NE555
Janos Nagy - 2008-29-2 13:06:00 PST -
Is there a problem with this circuit, or what am I missing?
Q1 switches to GND (0 volts) the inverting input of the OpAmp IC1 that is already (always) at the same (0 volts) potential.
How does this discharge C1?
It can be fixed of course.
Yitzhak Friedman - 2008-27-2 04:08:00 PST
