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DIY builds its own TTL oscillator circuit
Here is an LC oscillating circuit designed to drive a TTL circuit with a frequency of 1 Hz. This low-frequency oscillation can be synchronized with an earphone speaker and an LED, allowing users to both hear and visually observe the circuit's oscillation. It serves as an excellent starting point for electronics enthusiasts who want to build their own TTL circuits.
Oscillating circuits are among the most fundamental in analog electronics. For more information on their working principles, refer to "The Foundation of Analog Electronic Technology" by Tong Shibai and Hua Chengying, published by Higher Education Press.
This article outlines the process of designing an oscillating circuit from scratch. Each step of the design is carefully executed, with experience gained during the process guiding the next stage. The goal is to successfully design and fabricate a functional oscillating circuit.
1. **Common-Emitter Amplifier Circuit (Working Circuit)**
The transistor used here is the 2N3904. Base resistor Rb and collector resistor Rc are used to convert voltage changes into current changes. Choosing appropriate values for VBB, VCC, Rb, and Rc ensures that the transistor operates in the active region.
2. **Direct-Coupled Common-Emitter Amplifier**
In this configuration, the base and collector power supplies are combined, creating a direct-coupled amplifier.
3. **Operating Point Stabilization Circuit**
Adding an emitter resistor Re introduces negative feedback, helping stabilize the operating point against temperature variations.
4. **Capacitive Coupling Circuit**
Capacitors C1 and C2 are added between the signal source and the amplifier, as well as between the amplifier and the load, forming a capacitive coupling system. A bypass capacitor Ce is also included to short AC signals to ground.
5. **Capacitive Feedback Oscillator**
By connecting the two capacitors of the LC circuit to the three terminals of the transistor, a feedback oscillator is formed. This circuit uses LC resonance, transistor amplification, and voltage-series feedback.
**Oscillation Frequency Formula:**
$ f = \frac{1}{2\pi\sqrt{LC}} $
To achieve a 1 Hz frequency, larger components are needed: 5 mF for C1 and C2, and 10 mH for L.
The oscillation condition must also be satisfied:
$ \beta \cdot r_{be} > R_L $
Using this formula, we calculate the collector resistor Rc as 180 Ω.
6. **Building the 1 Hz Oscillator**
Components should be carefully adjusted, ideally using a breadboard or a rice board for easy modifications. Key tips include:
- Use a multimeter to monitor voltage and current changes during adjustments.
- Connect an LED across C1 to visualize the voltage variation; a 10 Ω resistor in series helps control brightness.
- Use a small speaker in parallel with C1, connected through a 100 Ω resistor to avoid excessive current. You’ll hear a soft “plop†sound, indicating oscillation.
For low-amplitude oscillations, the LED may not light up, but the speaker will still pick up the faint sound. This makes it easier to adjust the circuit and fine-tune the frequency.
Other component values:
- Rb1 = 75 kΩ, Rb2 = 200 kΩ
- Re = 10 Ω
- C3 = 220 μF (for 1 Hz)
- Ce = Two 10 mF capacitors in series
7. **Final Circuit with Sound and Light Output**
The completed 1 Hz oscillator includes a speaker and LED as visual and auditory indicators. The physical setup demonstrates the circuit’s operation effectively.
[Physical image of the 6V power supply and 1 Hz oscillator]
[Image of the assembled circuit with LED and speaker]
This project provides a hands-on introduction to oscillating circuits and is a great way for beginners to explore electronics.