A compact photoelectric smoke-detection system can be constructed using the Cherry Semiconductor CS235 integrated circuit. A pulsed infrared LED and a silicon photodiode are used together as the light source and detector pair. With the addition of a few discrete components, you can create a sensitive and reliable smoke detector. By cascading a number of these detector modules, a complete house smoke-detector systems can be fabricated.
The detector system consumes very little power. It pulses the system once every 10 seconds for 20 milliseconds. During the second half of the 20-ms period the LED is pulsed and the detector samples for a smoke condition. After the first alarm signal is detected a sample rate increases to a 2-second interval. After three consecutive alarm level samples are accumulated, the logic signal drives the output latch on pin 3. This synchronous detection method has very high noise rejection and provides excellent reliability.
The internal latch is capable of sinking 100mA of current and will drive a low-current relay directly. The CS235 operates from a 12Vdc supply. The latch current is determined by resistor RL, as shown in Figure, and depends upon the relay that you choose in your particular system.
The smoke detector system uses a two-ground system. A high-level ground is present on pin 4 and the low-level ground is connected on the voltage divider on pin 5. The CS235 smoke detector system is easy to construct and will provide a reliable protection system. The smoke detector can be operated on a 12V battery trickled-charged from a 110V source.
Photoelectric Smoke Detedor Parts List
| Quantity | Part | Description |
| 1 | R1 | 20 MΩ, ¼W resistor |
| 1 | R2 | 75 KΩ, ¼W resistor |
| 1 | R3 | 6 KΩ, ¼W resistor |
| 1 | R4 | 510 Ω, ¼W resistor |
| 1 | R5 | 100 KΩ, trim pot |
| 1 | R6 | 18 KΩ, ¼W resistor |
| 1 | R7 | 12 MΩ, ¼W resistor |
| 1 | R8 | 36 Ω, ¼W resistor |
| 1 | R9 | 270 KΩ, ¼W resistor |
| 2 | R10, R11 | 150 KΩ, ¼W resistor |
| 1 | RL | Current Limiter Adjusted for 100 mA |
| 1 | C1 | 0.47 μF, 25V capacitor |
| 1 | C2 | 150 μF, 25V capacitor |
| 1 | C3 | 0.0068 μF, 25V capacitor (disk) |
| 2 | C4, C5 | 0.47 μF, 25V capacitor |
| 2 | C6, C7 | 0.001 μF, 25V capacitor (disk) |
| 1 | D1 | Status LED |
| 1 | D2 | Infra Red LED |
| 1 | D3 | Photodiode |
| 1 | U1 | CS235 (Cherry Semiconductor) |
| 1 | RL-1 | 9-12Vdc SPST relay |
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