LONG PERIOD TIMER

Description: This circuit utilizes the 7240 CMOS programmable timer chip (IC3), which has an accuracy of 0.5 percent. It features a time-base generator, with the frequency determined by a resistor and a capacitor. The basic time period is RC seconds; with a maximum resistor value of 10 MΩ and a maximum capacitor value of 1000 µF, the maximum time period can reach 10,000 seconds (approximately 2.8 hours). IC3 also includes an eight-stage binary divider chain, allowing for a total period of 27 times the basic period, exceeding 14 days. In the schematic, S1 represents an optional power switch, while S2 is the switch used to initiate timing. Closing S2 generates a brief low pulse sent to pin 11 of the timer IC3, which starts the timing process. The output of the timer remains high but transitions to low for the duration of the timing interval, which is selectable via a rotary switch S3. Pins 1 to 8 of IC3 provide outputs from the eight-stage divider chain. Upon resetting the counter, all outputs go high, and during timing, they follow an inverted binary sequence. Resistor R5 connects the output to the RESET terminal (pin 10), ensuring the counter resets at the end of the timing interval. The initial logic stage comprises two NOR gates and a NAND gate configured as an inverter (part of IC1, 4011) to detect the two alarm states. A high output on either pin 8 or pin 9 results in a low output from gate IC2c (pin 10), driving the RESET input of IC4 low. IC4 is a 14-stage counter with its own oscillator that activates when the RESET is low. The oscillator operates at approximately 25 kHz, producing various frequencies: 1.6 kHz at pin 7 (high-pitched note), 200 Hz at pin 6 (low-pitched note), 6 Hz at pin 1 (fast bleeping), and 1.5 Hz at pin 3 (slow beeping). The remaining logic generates the fast high-pitched bleep signal for the red LED (D1) through transistor TR2, and the slower, low-pitched bleep for the green LED (D2) via TR3. Both signals are routed to the speaker LS1 through transistor TR1.

The circuit is designed to provide a versatile timing mechanism suitable for various applications, leveraging the capabilities of the 7240 CMOS timer chip. The time-base generator operates based on the RC time constant, allowing for precise timing intervals that can be adjusted according to the needs of the user. The maximum timing duration, extending to 14 days, is particularly beneficial for applications requiring long-term timing solutions.

The inclusion of an eight-stage binary divider enables the circuit to produce multiple timing outputs, each representing a different state of the timing process. The use of a rotary switch for interval selection allows for user-friendly adjustments, facilitating quick changes to timing settings without the need for complex reconfiguration.

The logic stage, consisting of NOR and NAND gates, ensures reliable detection of alarm states, providing an efficient means to trigger the reset functionality of the 14-stage counter. This counter, equipped with its oscillator, produces a variety of audio signals that can be used for alerting or notification purposes. The ability to generate both high-pitched and low-pitched tones, as well as varied frequencies for fast and slow beeping, adds versatility to the circuit's output capabilities.

Transistors TR1, TR2, and TR3 serve as amplifying components that drive the LEDs and speaker, ensuring that the audio and visual outputs are sufficiently powered for effective signaling. Overall, this circuit exemplifies a well-integrated design that combines timing, logic, and output functionalities in a coherent manner, making it suitable for applications ranging from alarms to timers in various electronic systems.This circuit is based on the 7240 CMOS programmable timer chip (IC3), which has an accuracy of 0. 5 percent. It contains a time-base generator, the frequency of which is decided by a resistor and a capacitor. The basic time period is RC seconds; so, given that the maximum value of R is 10 M © and the maximum value of C is 1000 F, the maximum time period is 10, 000 seconds, (2. 8 hours) IC3 also has an eight-stage binary divider chain. The total period available is 27 times the above, which is just over 14 days. In this figure, S1 is an optional power switch. S2 is the switch which is closed to initiate timing. Closing S2 generates a brief low pulse that goes to pin 11 of the timer IC3 and starts the timing. The output of the timer is normally high, but it goes low for the whole of the timing interval. The length of the interval is selected by a rotary switch S3. Pins 1 to 8 of IC3 are the outputs from the eight-stage divider chain. When the counter is reset, they all go high; while timing, they go through an inverted binary sequence. R5 connects the output to the RESET terminal (pin 10), so the counter is reset at the end of the interval.

The first stage of the logic consists of two NOR gates and a NAND gate wired as an inverter (part of IC1, 4011). These gates detect the two alarm states. A high output on either pin 8 or pin 9 causes a low output from gate IC2c (pin 10). This makes the RESET input of IC4 low. IC4 is a 14-stage counter with its own oscillator, which begins to oscillate when the RESET is made low.

The oscillator has a period of about 25 kHz, which is divided down to produce 1. 6 kHz at pin 7 (high-pitched note), 200 Hz at pin 6 (low-pitched note), 6 Hz at pin 1 (fast bleeping), and 1. 5 Hz at pin 3 (slow beeping). The remainder of the logic consists of gates producing the fast high-pitched bleep signal, which goes to the red LED (D1) by way of transistor TR2, and the slower, low-pitched bleep, which goes to the green LED (D2) by way of TR3.

Both signals go to the speaker LS1 by way of TR1.