| ??? 03/05/06 13:53 Modified: 03/05/06 15:19 Read: times |
#111287 - 74121 versus CMOS555 Responding to: ???'s previous message |
Richard said:
the '121 is a non-retriggerable one-shot. If you make a transition on its trigger input, it fires. However, until the next inactive-to-active transition, it ignores the level of the trigger. The 74121 would be a nice candidat. Unfortunately, it isn't suited to produce stable and precise pulse lengths of up to 10sec: As the maximum timing resistor is 40kOhm, according to datasheet, a timing cap of 360µF would be needed for a 10sec pulse. Such a huge capacitance can only be provided by an aluminium electrolytic. But these caps are well known for their instability and drift, not even mentioning the manufacturing tolerances. Only a foil cap is suited to produce stable pulses! Also, variation in output pulse width versus supply voltage and versus temperature of 74121 is much higher compared to CMOS555. More, threshold voltage of the timing section is much lower compared to CMOS555. Also, internal reference voltage cannot be filtered externally as it can with the CMOS555. These make the 74121 much more susceptible against noise on supply voltage and ground. This results in a wider spread distribution of output pulse widths. Another disadvantage of 74121 is, that supply voltage is limited to 5V. With the CMOS555 supply voltage can vary between about 2V and 18V, depending on derivative. So, Suresh can use one single supply for the CMOS555 and the dispenser. Kai |
