Overview
This lecture explains how to build a basic oscillating circuit using a relay and capacitor, suitable for triggering a speaker (to make sound) or an LED (to blink).
Introduction to Oscillating Circuits
- An oscillating circuit switches automatically between on and off states, like a blinking LED or beeping sound.
- Many designs exist, but a relay oscillator is the simplest, though not the most stable or robust.
Relay Oscillator Circuit Design
- Uses a double pole double throw (DPDT) relay for switching.
- Connect positive battery terminal to one relay coil terminal; negative to a switching terminal that defaults to connected when unpowered.
- When first powered, current flows through the switching part, energizing the electromagnet, which changes the relay state and cuts off current to the coil.
- As current stops, the relay resets, restoring the connection and restarting the cycle.
- This rapid oscillation can damage relays and is too fast for many uses.
Slowing Oscillation with a Capacitor
- Adding an electrolytic capacitor across relay coil terminals stores and releases energy, maintaining coil power briefly after the switch, slowing oscillation.
- Higher capacitor values slow the oscillation and protect the relay.
Attaching Output Devices
- Output can be an LED (with resistor) for blinking or a speaker for sound.
- Connect the output device to the relay’s other switch contacts.
- Add a manual switch in series to control when the speaker or LED is active.
Demonstration and Experimentation
- Use a 5V power supply and ensure your speaker matches this voltage.
- Start with a 1,000 microfarad capacitor; higher values prevent relay damage and create audible sounds.
- Decreasing capacitance (e.g., 470 microfarad) increases oscillation frequency, resulting in a higher-pitched sound.
Key Terms & Definitions
- Relay Oscillator — Circuit using a relay to alternate between on and off states automatically.
- DPDT Relay — Double Pole Double Throw relay, can switch two circuits at once.
- Electrolytic Capacitor — Polarized capacitor storing electrical energy, affects oscillation speed.
- Oscillation Frequency — The rate at which the relay switches, controlled by capacitance.
Action Items / Next Steps
- Experiment with different capacitor values to observe changes in oscillation.
- Prepare for the next lecture on integrated circuit (IC) oscillators for improved performance.
- Ensure you understand relay and capacitor connections before applying power.