# Noob Question, 74HC240 and Solar Cells

My idea is to start soldering and I would start with something quite simple. I chose the project (https://cdn.makezine.com/make/08/pummer.pdf), a pummer with solar cell charging up a supercapacitor and then starting to pum a led when the capacitor is full. In the schematic and explanatiuon it is suggested a solar cell giving 20mA at 3V. Now, I quite understand the schematic, it isn't something too complicated. What I do not understand is the solar cell choice. Looking at the 74HC240 datasheet (http://assets.nexperia.cn/documents/data-sheet/74HC_HCT240.pdf) I see the following data: Input Clamping power: +/- 20mA Supply Current: 70mA. What happens if I use a solar cell with a lower current intensity? I found one with Isc=13.5µA. Does this suits the circuit or it will be a bad choice? Maybe I can use two solar cell with that specs in parallel? Thanks Ale

• @ChrisStratton Sorry for the first question posted, Now I edited the question not to be dependent on external links. Ale – mant0 Oct 28 '20 at 15:25
• From the sound of it, you're looking at a solar cell meant for use as a sensor not really for power generation. However, you're also misreading the buffer spec - you are looking at a maximum, not typical. A CMOS part has almost no quiescent current draw, but really only uses power charging and discharging capacitances. However in microamp territory, this part's possibly relatively high static power consumption may challenge that wimpy solar cell. Look for a cell of several square centimeters area. – Chris Stratton Oct 28 '20 at 15:49
• 13.5 uA is over 1000 times less than the recommended 20 mA. – Chris Stratton Oct 28 '20 at 15:54
• @ChrisStratton Thank you for your help. Reading at your comments i do understand that I need something way bigger, or with higher A to match what the guide requires. I do know the order of magnitude in difference between the one proposed by the guide and I proposed. I was wondering what can happen supplying that few current. Thanks – mant0 Oct 28 '20 at 15:55