Membrane Keyboard Scan Circuit for High-Resistance Carbon Traces
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Solution Overview
Problem
Conventional membrane keyboard scan circuits designed for silver paste membranes are not suitable for carbon paste membranes due to the significantly higher resistance of scan lines, leading to incorrect voltage levels and interference issues.
Innovation Solution
The membrane keyboard scan circuit includes a row scan-port circuit with two select branches and a column scan-port circuit, allowing for adaptation to either silver paste or carbon paste membranes by adjusting pull-up resistors and input voltage control circuits to match the specific resistance and voltage thresholds of each type, ensuring accurate key detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If carbon paste membrane is used to lower manufacturing cost, then manufacturing cost is reduced, but scan line resistance increases significantly (over 200K Ohm compared to less than 5K Ohm for silver paste)
Solution Approach 1:
The patent changes the electrical parameters of the scan circuit by introducing a voltage divider circuit consisting of a first resistor and a second resistor. This circuit transforms the high resistance signal from the carbon paste membrane into a voltage level that can be properly detected by the microcontroller, thereby resolving the reliability issue caused by high scan line resistance while maintaining the use of cost-effective carbon paste material.
Solution Approach 2:
The voltage divider circuit acts as an intermediary between the high-resistance carbon paste membrane and the microcontroller input. The first and second resistors form this intermediary circuit that adapts the impedance mismatch, enabling reliable signal detection despite the significant resistance difference between carbon paste and the microcontroller's input impedance.
2Reliability
If conventional scan circuit for silver paste is used, then voltage levels are appropriate, but the circuit cannot accommodate carbon paste membranes with higher resistance
Solution Approach 1:
The patent creates a universal scan circuit design that can accommodate both silver paste and carbon paste membranes. By incorporating the voltage divider circuit with configurable resistors, the same hardware design works reliably for both membrane types, eliminating the need for separate circuit designs and improving adaptability across different membrane technologies.
Solution Approach 2:
The circuit uses adjustable resistor values in the voltage divider to adapt to different membrane resistance characteristics. This parameter adjustment capability allows the single circuit design to maintain appropriate voltage levels for both low-resistance silver paste membranes and high-resistance carbon paste membranes, achieving versatility without sacrificing voltage level accuracy.
3Ease of manufacture
If high resistance carbon paste membrane is used, then manufacturing cost decreases, but anti-interference characteristics deteriorate
Solution Approach 1:
The voltage divider circuit transforms the weak, high-impedance signal from the carbon paste membrane into a stronger, lower-impedance voltage signal that is more resistant to noise and interference. By changing the electrical parameters through the resistor network, the circuit improves signal integrity and anti-interference characteristics while maintaining the use of cost-effective carbon paste material.
Data Source
AI summary
A membrane keyboard scan circuit, a scan method and a keyboard having the same are provided. A row scan-port circuit of the scan circuit comprises a first and a second select branches corresponding to a first type of the membrane keyboard and a second type of the membrane keyboard respectively. A column scan-port circuit of the scan circuit comprises a fifth switch connected between a power supply voltage and an output port of the row scan-port circuit and a sixth switch connected between a ground line and the output port. The column scan-port circuit performs a scan output process to provide a row-scan timing signal to a key circuit of the membrane keyboard such that one of the first and the second select branches generates a sensing signal when the key circuit is triggered.