Hi all eletronics seniors,
I am new here. I am trying to multiplex ten 7-segment common cathode.
here is my circuit explanation:
assume that each segment need 5mA to turn on (forward biased voltage = 1.7), 10 digit means I need 50mA, right?
I have used seven 2N2222A as 7-segment drive. the collector is to 9Vcc with 140ohms [(9v - 1.7v)/50mA]. the output also to 7-segment [ie. when BJT off, segment lit on; when BJT ON, segment off]. The emmitter is to GND. The base is connected to PIC output[2.5v] with a diode in series with resistor of 2.2kohm [500uA]. The diode is to protect the PIC from damage if the BJT damaged. My questions are:
1. I run simulation and it seems ok.but when i build the circuit, i spoiled every IC link to it including PIC. By the way, i used 4017 counter with 7404 inverter to turn each digit ON/OFF. Is it because I didn't put heat sink at the common cathode of 7-segment?
2. 500uA on base current is it too big? will it spoiled the BJT?
3. I found that even when BJT ON [ segment should be OFF ], but i can still see the dim color. is there anyway to make it off?
4. I have tried to connect the emitter of each BJT to each 7-segment, it turns out to be very dim..[because i don't want to invert software coding]. I thought by making sure each segment have 5mA would be sufficiently bright enough?
5. PortA of my 877 seems to source 2.5v instead of 5v, why is that so? I am confused over it.
I really apreciate your feedbacks.
I am new here. I am trying to multiplex ten 7-segment common cathode.
here is my circuit explanation:
assume that each segment need 5mA to turn on (forward biased voltage = 1.7), 10 digit means I need 50mA, right?
I have used seven 2N2222A as 7-segment drive. the collector is to 9Vcc with 140ohms [(9v - 1.7v)/50mA]. the output also to 7-segment [ie. when BJT off, segment lit on; when BJT ON, segment off]. The emmitter is to GND. The base is connected to PIC output[2.5v] with a diode in series with resistor of 2.2kohm [500uA]. The diode is to protect the PIC from damage if the BJT damaged. My questions are:
1. I run simulation and it seems ok.but when i build the circuit, i spoiled every IC link to it including PIC. By the way, i used 4017 counter with 7404 inverter to turn each digit ON/OFF. Is it because I didn't put heat sink at the common cathode of 7-segment?
2. 500uA on base current is it too big? will it spoiled the BJT?
3. I found that even when BJT ON [ segment should be OFF ], but i can still see the dim color. is there anyway to make it off?
4. I have tried to connect the emitter of each BJT to each 7-segment, it turns out to be very dim..[because i don't want to invert software coding]. I thought by making sure each segment have 5mA would be sufficiently bright enough?
5. PortA of my 877 seems to source 2.5v instead of 5v, why is that so? I am confused over it.
I really apreciate your feedbacks.