Design Ideas: August 15, 1996

The circuit in Figure 1 allows you to measure light using a regular digital voltmeter. The TSL230B (IC₁) is a programmable light-to-frequency converter whose output frequency is proportional to the light level. Inputs S₀ and S₁ set the sensitivity of IC₁ to 1X, 10X, or 100X. S₂ and S₃ control the internal programmable divider (see the tables in the figure). When S₀ is high and S₁, S₂, and S₃ are low, a 1V output is equivalent to an irradiance of 1 mV/cm².

IC_2A, IC_2B, R₁, R₂, C₁, and a 4-MHz crystal form a stable oscillator. This oscillator's output clocks IC₃'s 12-bit binary counter through IC_2D. The output of IC₁ resets the counter through C₂ and R₃. Once IC₃ resets, its Q₉ output is low, and the output of IC_2C is high. IC_2D's gate is open, which allows IC₃ to count. When the Q₉ output goes high, it blocks the gate to disable the clock until IC₃ resets again. Every time IC₃ resets, the Q₈ output of IC₃ forms a 32-µsec pulse. R₄ and R₅ divide and C₃ averages this output to drive a digital voltmeter. The less light, the longer the reset time and the lower the meter reading.

When S_O and S₃ are low and S₁ and S₂ are high, tests reveal the results in Table 1. IC₁ has a 1 to 10,000 measurement range with full-scale resolution. The full-scale linearity of this IC over the entire range is 0.5%. Measurement time takes less than 100 µsec. Other advantages include a 0.1%/°C temperature coefficient and a 0.5%/V supply-voltage sensitivity. (DI #1908)