--- title: "Voltage controlled oscillator using 555" subject: "Electronics Devices and Circuit" chapter: 5 key: "semester_3/electronics_devices_and_circuit:vco-555" source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/vco-555 site: BCT Engineering --- # Voltage controlled oscillator using 555 Electronics Devices and Circuit · Chapter 5: Signal Generator and Waveform-Shaping Circuits - Asked in: 2076 Chaitra, 6 marks; 2076 Ashwin, 8 marks; 2074 Chaitra, 6 marks; 2071 Chaitra, 6 marks (asked 4 times) - Video: [IC 555 as Voltage Controlled Oscilltor](https://www.youtube.com/watch?v=XtJAemoc8BI) — Ekeeda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/vco-555 - Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-5 ## Question Draw voltage controlled oscillator circuit using IC 555 and derive expression for frequency of oscillation. ## Short answer Control voltage Vc at pin 5 sets thresholds Vc and Vc/2. Charging time depends on Vc; discharging time tL = 0.693 RB C. Frequency f = 1/{ (RA+RB)C ln[(VCC–Vc/2)/(VCC–Vc)] + 0.693 RB C }. Increasing Vc lowers frequency. ## Answer A voltage controlled oscillator (VCO) gives an output frequency that depends on an applied control voltage. In the 555 timer, the control voltage is applied at pin 5 (CONTROL), which sets the comparator thresholds. #### Circuit ```text +VCC +------+------+ | | | [RA] (8)(4) | | | +----(7) | | [RB] (3) -----> output | +----(6) +----(2) | [C] | GND Vc o----(5) control voltage input (e.g. from a pot or source) ``` Same as an astable multivibrator, but pin 5 is driven by an external voltage Vc (instead of the 0.01 µF capacitor to ground). #### Derivation With control voltage Vc at pin 5: the upper threshold = Vc and the lower threshold = Vc/2 (the internal divider of equal resistors). **Charging (HIGH):** The capacitor charges toward VCC through (RA + RB) from Vc/2 to Vc: Vc = VCC − (VCC − Vc/2) e^(−tH/(RA+RB)C) **tH = (RA + RB) C ln[(VCC − Vc/2)/(VCC − Vc)]** **Discharging (LOW):** The capacitor discharges through RB from Vc to Vc/2: Vc/2 = Vc e^(−tL/(RB C)) **tL = RB C ln 2 = 0.693 RB C** **Frequency:** **f = 1/(tH + tL) = 1 / { (RA + RB) C ln[(VCC − Vc/2)/(VCC − Vc)] + 0.693 RB C }** When Vc = 2VCC/3 (pin 5 unconnected): (VCC − VCC/3)/(VCC − 2VCC/3) = 2, so tH = 0.693 (RA + RB) C and f = 1.44/((RA + 2RB)C), the normal astable result. **Behaviour:** Increasing Vc raises both thresholds, so charging takes longer and the frequency decreases. Reducing Vc raises the frequency. The frequency is therefore controlled by Vc, which makes a VCO. Applications: FM generation, tone generators, FSK, PLLs. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is a 555 astable turned into a VCO? A. A control voltage is applied at pin 5 instead of the 0.01 µF capacitor B. A control voltage is applied at pin 4 (reset) C. RA is replaced by a diode D. The output pin 3 is fed back to pin 7 Answer: A. Pin 5 (CONTROL) sets the comparator thresholds, so driving it with Vc changes the frequency. (Key point: VCO control pin) 2. With control voltage Vc at pin 5, what are the upper and lower comparator thresholds? A. Vc and Vc/2 B. 2VCC/3 and VCC/3 C. VCC and Vc D. Vc/2 and Vc/4 Answer: A. The upper threshold equals Vc and the internal equal-resistor divider makes the lower threshold Vc/2. (Key point: Thresholds) 3. The capacitor discharges from Vc to Vc/2 through which resistance, and what is tL? A. RA + RB; tL = 0.693 (RA + RB) C B. RB; tL = 0.693 RB C C. RA; tL = 0.693 RA C D. RB; tL = 1.44 RB C Answer: B. Discharge is through RB into pin 7, and a halving of voltage gives tL = RB C ln 2 = 0.693 RB C. (Key point: Discharge time) 4. What happens to the output frequency when Vc is increased? A. Frequency increases because the capacitor swings less B. Frequency decreases because charging takes longer between higher thresholds C. Frequency is unchanged D. Output stops oscillating Answer: B. Raising Vc raises both thresholds so the capacitor must charge further, taking longer, hence lower frequency. 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