Hello,
I am working on a digital counter to go in a display case at my school. I have built the circuit and it is working well. The goal of the circuit is to count down 5 billion years, in tenths of a second. (it is estimated that the sun will swallow the earth in 5 billion years. much more on this on wikipedia) The display is composed of 20 7-segment displays. Only 11 of the digits will change in 300 years, so each of those is wired with a 7447 74190 pair and hooked to a clock source derived from the AC mains. The other 9 digits are hard wired to show the appropriate number. It is possible this could run for 5+ years. I realize when significant figures are taken into account this project is ridiculous. It is meant for illustrative/entertainment purposes.
My question is this; I would like to implement a battery backup for the current count, incase a janitor decides to unplug the display or the power goes out, etc. It is not a requirement for the counter to increment while on battery backup, only retain the current count. I have a few ideas, and am looking to see what the best route may be.
- have a small battery and a circuit to write the current count to eeprom when mains are disconnected, then turn off. display would always load current count from eeprom when AC mains are turned on.
- have a larger battery maintain power to all 11 74190's so they retain their count when AC mains are off. The batteries could also be trickle charged/maintained when the AC is on.
- have some sort of flash memory always have the current count stored.
I have some 12V sealed lead-acid motorcycle batteries that could prolly work. However I can't have any funky battery acid leaking all over the display case. Also the display case is not well ventilated.
Of course the project budget is very limited. We have a electronic parts store room that was very well equipped in the mid 80's and hasn't been touched since. no fancy all-in-one ICs, LOTS of logic ICs, transistors, resistors and capacitors.
your help is much appreciated!
thanks,
Skot
I am working on a digital counter to go in a display case at my school. I have built the circuit and it is working well. The goal of the circuit is to count down 5 billion years, in tenths of a second. (it is estimated that the sun will swallow the earth in 5 billion years. much more on this on wikipedia) The display is composed of 20 7-segment displays. Only 11 of the digits will change in 300 years, so each of those is wired with a 7447 74190 pair and hooked to a clock source derived from the AC mains. The other 9 digits are hard wired to show the appropriate number. It is possible this could run for 5+ years. I realize when significant figures are taken into account this project is ridiculous. It is meant for illustrative/entertainment purposes.
My question is this; I would like to implement a battery backup for the current count, incase a janitor decides to unplug the display or the power goes out, etc. It is not a requirement for the counter to increment while on battery backup, only retain the current count. I have a few ideas, and am looking to see what the best route may be.
- have a small battery and a circuit to write the current count to eeprom when mains are disconnected, then turn off. display would always load current count from eeprom when AC mains are turned on.
- have a larger battery maintain power to all 11 74190's so they retain their count when AC mains are off. The batteries could also be trickle charged/maintained when the AC is on.
- have some sort of flash memory always have the current count stored.
I have some 12V sealed lead-acid motorcycle batteries that could prolly work. However I can't have any funky battery acid leaking all over the display case. Also the display case is not well ventilated.
Of course the project budget is very limited. We have a electronic parts store room that was very well equipped in the mid 80's and hasn't been touched since. no fancy all-in-one ICs, LOTS of logic ICs, transistors, resistors and capacitors.
your help is much appreciated!
thanks,
Skot