Long Duration Timing Using the 555

July 2003

A long duration one-shot (monostable multivibrator) can be inexpensively and simply built using a 555 timer. If we need to keep something on (or off) for more than ten minutes and if accuracy is not paramount (if we can live with plus or minus many seconds) we can use the 555 for such timing purposes.

The choice of capacitor is very important. Since long duration timing entails very slow charging rates, the capacitor employed must have minimal leakage. Ordinary electrolytics while having the advantage of large capacitances have the unfortunate drawback of being very leaky. If we foolishly try to use these capacitors, they will probably never charge to 2/3 of the voltage required by pin6 to reset the 555 since the leakage rate will probably exceed the charging rate. Therefore, other types of capacitors with moderately high capacitances have to employed, such as tantalum capacitors.

One CMOS version of the 555 is National Semiconductor's LMC555. Together with a 10% tantalum capacitor and megaohm resistors we can assemble a one-shot that can output a pulse of more than 15 minutes.

In the circuit I've tested I used three 10Mohm resistors in series to reach a total of 30Mohms (I haven't been able to procure resistors greater than 10Mohms). I used a 22microfarad tantalum capacitor (10% tolerance). I used these components and connected them to the LMC555 to create a monostable. Output pulse was 15.5 minutes.

Output drive may be a problem with the CMOS version of the 555. Excessive voltage drop across V+ and pin3 will develop if you try to source more than 30mA even with a 12V power supply. The solution here is to drive a small signal NPN transistor (with a collector current capacity of 100 to 500mA depending on the load) instead and use the collector to sink current from the load.

The diagram below shows a complete circuit (sans power supply) that I'm currently using to switch a 2.2kW water heater.

heater timer circuit using the LMC555

R1 is a string of 10Mohm resistors in series. C1 is a 22microfarad tantalum capacitor (10% tolerance). LR is a red LED to indicate that the timer is on standby and that the heater is off. LY is a yellow LED that indicates the heater is on. OPTO is an optoioslator with a triac output (a MOC3021 in this case). TRIAC is a Teccor Q401E3 400V 1A triac that is used to switch on CON, a magnetic contactor (big brother of relays) whose N.O. (normally open) contacts are rated at 2.2kW. System voltage is 12VDC but it can be much less than this.

The LMC555 is wired in monostable mode such that when momentary switch ON is depressed pin2 goes low and triggers the timer to start charging C1. Simultaneously pin3 goes high and switches NPN transistor Q1 on. Q1 in turn switches OPTO and LY on. With trigger current avaiabable, TRIAC goes into its conducting state energizing contactor CON. With CON energized its contacts close thus allowing line current to flow into HEATER. When C1 finally reaches 2/3 of system voltage pin3 goes low and C1 is discharged via pin7. If switch OFF is depressed at anytime during the charging cycle, charging is aborted and everything is reset.

The 1uF capacitors connected to the two momentary switches are necessary to prevent line transients from triggering the 555. As explained above Q1 is necessary because the voltage output at pin3 of LMC555 drops significantly as source current reaches the tens of milliampere range. Its sink current capacity is significantly higher than its sourcing capacity. Since LR utilizes less than 10mA no output amplifier (transistor in this case) is necessary for the LR circuit