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NBN backup battery at 0. The game begins by pressing S7, causing LED1 to flash randomly one to three times and then remain on while the sounder generates a beep. Once LED1 is on, or the beep is generated, all four counters start counting up in milliseconds from 1 to 8000. Now each player should press their own pushbutton switches as fast as possible to stop their relevant counters. The fastest player to press their button wins. Then the player with the quickest finger or the minimum reaction time (the minimum counter value) is announced as the winner on the LCD screen (eg, “Winner: Player 4”). It’s possible to have multiple winners if the buttons are pressed simultaneously. In all three games, if a button is pressed too early (in the start status while LED1 is blinking), it will continue flashing, and the sounder will beep until the button is released. Then after another random flashing cycle, LED1 will stay on while a beep is made to signal to the players to press the play buttons to resume the game. If one or more buttons are not pressed within eight seconds (8000 milliseconds), the counters stop at 8001. The display will then clear to show “Timeout Error” on the first line and “Press start button to play again” on the third and fourth lines. To reset and restart the game, press S7. The software for this circuit can be downloaded from: siliconchip.com. au/Shop/6/6339 Mahmood Alimohammadi, Tehran, Iran. ($80) siliconchip.com.au Having recently had the NBN installed, I found that now I had two devices (the NBN modem and the wireless router) both powered by plugpacks and both dependent on mains power to keep working. We get the occasional blackout in my area, and without these devices, I have no internet connection on my tablets and other battery-powered devices. Reading past articles in Silicon Chip inspired me to design a simple backup power supply. The circuit operates as follows. The mains power supply is connected via CON1. I used one of the NBN plugpacks (12V DC). IC1 is a precision adjustable shunt regulator, but is being used here as an open-collector comparator with a precision voltage reference connected to one of its inputs. With voltage across CON1, transistor Q1 is switched on and it pulls the reference input of IC1 low. This prevents IC1’s output transistor from conducting, so Q2 is off, and the only path to output connector CON3 is from CON1 via schottky diode D1. LED1 is also lit as it is powered from the DC supply, indicating normal operation. When the plugpack voltage disappears, as long as the battery voltage is above 9.25V, IC1’s reference input is allowed to rise above 2.5V. So IC1 sinks current from its anode terminal, switching transistor Q2 on, supplying the output from the backup battery. Australia's electronics magazine Since I used a lithium-ion battery as the backup battery, I needed to be able to disconnect the output if the backup battery voltage fell below 9V. You would probably change the 27kW resistor to 33kW for a lead-acid battery, raising the cut-out threshold to 10.75V. The green LED indicates that plugpack power is available, and the red LED indicates when the output is supplied from the battery. If both LEDs are off, there is no output voltage. This circuit does not include any way for the battery to be charged, because where I live, power outages are infrequent and I am happy to recharge or swap the battery manually every few months. If you need to keep the battery charged, you can permanently connect a mains trickle charger (for lead-acid types) or lithium-ion maintenance charger to the battery. D1 should be a schottky diode of sufficient rating for the load current. Transistor Q2 can be any PNP transistor rated for the output current; something in a TO-126 or TO-220 package will do, and it does not have to be heatsinked. I built my version on stripboard and housed it in a small plastic box. Of course, this can be adapted for many other uses where a simple backup supply is required. Robert Budniak, Denistone, NSW. ($80) April 2022  91