Driving two transistors with same signal
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Posted by petrus bitbyter on May 10, 2008, 9:31 pm
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>> BobW wrote:
>>>>> Hi,
>>>>> >>>>> I have two common anode 7-segments displays, and only one wire to >>>>> drive them. I need to achieve the following : when the control signal >>>>> is +5V, display1 is ON and display2 is OFF. When the signal is 0V, >>>>> display1 is OFF and display2 is ON. When the signal is not connected >>>>> (high impedance), both displays are OFF. >>>>> >>>>> I tried using a NPN transistor for display1 and a PNP for display2, >>>>> connecting their bases together. It works when the driving signal is >>>>> present, but when the signal is floating current flows from the base >>>>> of the PNP to the base of the NPN and both transistors turn each other >>>>> on, resulting in both displays being ON. >>>>> >>>>> The circuit that didn't work : >>>>> >>>>> VCC >>>>> + >>>>> | >>>>> | >>>>> ___ |< >>>>> o---------------------|___|--| >>>>> | |\ >>>>> | VCC | >>>>> | + | >>>>> | | Display2 >>>>> | | | >>>>> | ___ |/ | >>>>> Input---o---|___|--| GND >>>>> |> >>>>> | >>>>> | >>>>> Display1 >>>>> | >>>>> | >>>>> GND >>>>> >>>>> >>>>> >>>>> Is there a way to achieve this ? >>>>> >>>>> Thanks. >>>> Try this: >>>> 5V >>>> | 5V >>>> R5 | >>>> | |<--- >>>> |/-------------| Q3 >>>> ----| Q1 |\--------- (to display 1 and then to GND) >>>> | |>-- 5V >>>> | | | >>>> | | R3 >>>> | | | >>>> ---R1-----R2---------- >>>> | | | >>>> | | R4 >>>> | |<--- | >>>> |--| Q2 GND >>>> |\ >>>> | |/--------- (to display 2 and then to 5V) >>>> -------------| Q4 >>>> | |>---- >>>> R6 | >>>> | GND >>>> GND >>>> >>>> Q1 and Q2 form a comparator so that when the input is floating they >>>> will both be off. R3 and R4 set the input threshold. >>>> >>>> When the input is high (5V) then Q1 will be on and that will turn on >>>> Q3. When the input is low (GND) then Q2 will be on and that will turn >>>> on Q4. >>>> >>>> You can figure out the resistor values. They shouldn't be too critical, >>>> but R3 and R4 need to be small enough to ensure enough drive for the >>>> four transistors. >>>> >>>> I hope I got the ascii art right as I had to compose it in a separate >>>> word processor. >>>> >>>> Bob >>>> -- >>>> == NOTE: I automatically delete all Google Group posts due to >>>> uncontrolled SPAM == >>> >>> I see, now, that your displays are both common anode. You'll need to add >>> another PNP (common emitter mode) driven by Q4. >>> >>> As Monica Lewinsky used to say, "Close, but no cigar." >>> >>> Bob >>
>> *gasp* 5 transistors needed to do what seemed simple at first glance ... >> >> I don't quite understand what R5 and R6 are for, when Q1 and Q2 do not >> conduct, the base current of Q3 and Q4 would be zero so the ressitors do >> not seem necessary ? >> >> Well I guess I could just try it and see if it works :) >
> > Didn't you see my four transistors, four resistors solution yesterday? > > +--------+------------------+-------+--Vcc > | | | | > ___ |< | ___ |/ | > in----+--|___|---| | +--|___|--| | > | |\ | | |> | > | | | | | | > | | |< | | |< > | +------| | +-----| > | | |\ | | |\ > | .-. | | .-. | > | | | .---. | | | .---. > | | | | | | | | | | > | '-' | D | | '-' | D | > | | | | | | | | > | | '---' | | '---' > | | | | | | > ------)------------+--------+------)-----------+-------+--GND > | | > +----------------------------+ > created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de > > > petrus bitbyter > Oops, checking out my own schematic I found it wrong. The one below is good. I added a diode to make the voltage loss for both displays more equal. If that loss is not acceptable you can use a five transistor variant shown by someone else already. I added similar schematic below. +--------+------------------+-------+--Vcc | | | | ___ |< V .-. | in----+--|___|---| - | | | | |\ | | | | | | | '-' | | | |< | |/ | +------| +-----| | | |\ | |>
| .-. | | |
| | | .---. ___ |/ .---. | | | | | +--|___|--| | | | '-' | D | | |> | D |
| | | | | | | |
| | '---' | '---' | | | | | | ------)------------+--------+------)-----------+-------+--GND | | +----------------------------+ +------+---------------------+------+--Vcc | | | | ___ |< | |< | in---+--|___|--| | +--| | | |\ | | |\ | | | | .-. | |< | | |< | | +----| | +----| | | | |\ | | |\ '-' | | | .-. | | | | | | | .---. | | .---. | | | | | ___ |/ .-. | | | '-' | D | +--|___|--| | | | D | | | | | | |> | | | |
| | '---' | | '-' '---'
| | | | | | | -----)-----------+------+----)-----------+----+------+--GND | | +-----------------------+ created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de For a a Vcc of 5V all resistors can be 4k7. Transistors are general purpose 2N3904, 2N3906 or BC550, BC560. petrus bitbyter | |||||||||||||||||||||||||
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Posted by vic on May 10, 2008, 9:39 pm
Please log in for more thread options > Didn't you see my four transistors, four resistors solution yesterday?
> > +--------+------------------+-------+--Vcc > | | | | > ___ |< | ___ |/ | > in----+--|___|---| | +--|___|--| | > | |\ | | |> | > | | | | | | > | | |< | | |< > | +------| | +-----| > | | |\ | | |\ > | .-. | | .-. | > | | | .---. | | | .---. > | | | | | | | | | | > | '-' | D | | '-' | D | > | | | | | | | | > | | '---' | | '---' > | | | | | | > ------)------------+--------+------)-----------+-------+--GND > | | > +----------------------------+ > created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de > Sorry I missed your post. Looks like a good solution. Thanks ! | |||||||||||||||||||||||||
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Posted by petrus bitbyter on May 11, 2008, 4:14 pm
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> petrus bitbyter wrote:
>> Didn't you see my four transistors, four resistors solution yesterday?
>> >> +--------+------------------+-------+--Vcc >> | | | | >> ___ |< | ___ |/ | >> in----+--|___|---| | +--|___|--| | >> | |\ | | |> | >> | | | | | | >> | | |< | | |< >> | +------| | +-----| >> | | |\ | | |\ >> | .-. | | .-. | >> | | | .---. | | | .---. >> | | | | | | | | | | >> | '-' | D | | '-' | D | >> | | | | | | | | >> | | '---' | | '---' >> | | | | | | >> ------)------------+--------+------)-----------+-------+--GND >> | | >> +----------------------------+ >> created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de >> >
> > Sorry I missed your post. Looks like a good solution. Thanks ! Be aware that this schematic does not work. By modifiing my first schematic (with one display connected to Vcc) to the second (with both diplays connected to Gnd) I made a mistake. I posted the right schematic few minutes before your post. petrus bitbyter | |||||||||||||||||||||||||
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Posted by John Fields on May 10, 2008, 9:35 am
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>Hi,
> >I have two common anode 7-segments displays, and only one wire to drive >them. I need to achieve the following : when the control signal is +5V, >display1 is ON and display2 is OFF. When the signal is 0V, display1 is >OFF and display2 is ON. When the signal is not connected (high >impedance), both displays are OFF. > >I tried using a NPN transistor for display1 and a PNP for display2, >connecting their bases together. It works when the driving signal is >present, but when the signal is floating current flows from the base of >the PNP to the base of the NPN and both transistors turn each other on, >resulting in both displays being ON. > >The circuit that didn't work : > > VCC > + > | > | > ___ |< > o---------------------|___|--| > | |\ > | VCC | > | + | > | | Display2 > | | | > | ___ |/ | > Input---o---|___|--| GND > |> > | > | > Display1 > | > | > GND > > > >Is there a way to achieve this ? --- Yes. View in Courier: +5V>-------+-----------+-----------+-----------+
| | | |
[100k] [100K] | | | | | E +-----------|-----------|---------B 2N3906 | | | C | | E | | +--[10K]--B 2N3906 | [10k] | C | | | | | | C |A |A VIN>-------+--[10k]--B 2N3904 [DISPLAY] [DISPLAY]
E | |
| [150R] [150R] | | | GND>-------------------+-----------+-----------+
Version 4 SHEET 1 880 680 WIRE -112 -128 -272 -128 WIRE 144 -128 -112 -128 WIRE 384 -128 144 -128 WIRE 624 -128 384 -128 WIRE -112 -80 -112 -128 WIRE 144 -80 144 -128 WIRE 624 0 624 -128 WIRE -112 48 -112 0 WIRE 560 48 -112 48 WIRE 384 96 384 -128 WIRE -112 112 -112 48 WIRE 144 144 144 0 WIRE 192 144 144 144 WIRE 320 144 272 144 WIRE 144 176 144 144 WIRE 624 192 624 96 WIRE -112 224 -112 192 WIRE -32 224 -112 224 WIRE 80 224 48 224 WIRE -272 256 -272 -128 WIRE -112 256 -112 224 WIRE 384 288 384 192 WIRE 624 288 624 256 WIRE 384 384 384 352 WIRE -272 512 -272 336 WIRE -112 512 -112 336 WIRE -112 512 -272 512 WIRE 144 512 144 272 WIRE 144 512 -112 512 WIRE 384 512 384 464 WIRE 384 512 144 512 WIRE 624 512 624 368 WIRE 624 512 384 512 WIRE -272 592 -272 512 FLAG -272 592 0 SYMBOL pnp 560 96 M180 SYMATTR InstName Q1 SYMATTR Value 2N3906 SYMBOL LED 608 192 R0 SYMATTR InstName D1 SYMATTR Value QTLP690C SYMATTR Description Diode SYMATTR Type diode SYMBOL res 608 272 R0 SYMATTR InstName R1 SYMATTR Value 150 SYMBOL voltage -272 240 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL res -128 -96 R0 SYMATTR InstName R2 SYMATTR Value 100k SYMBOL voltage -112 240 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value PULSE(0 5 0 1e-6 1e-6 .1 .2) SYMBOL res -128 96 R0 SYMATTR InstName R3 SYMATTR Value 10k SYMBOL res 64 208 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R4 SYMATTR Value 10k SYMBOL npn 80 176 R0 SYMATTR InstName Q2 SYMATTR Value 2N3904 SYMBOL res 128 -96 R0 SYMATTR InstName R5 SYMATTR Value 100k SYMBOL pnp 320 192 M180 SYMATTR InstName Q3 SYMATTR Value 2N3906 SYMBOL LED 368 288 R0 SYMATTR InstName D2 SYMATTR Value QTLP690C SYMATTR Description Diode SYMATTR Type diode SYMBOL res 368 368 R0 SYMATTR InstName R6 SYMATTR Value 150 SYMBOL res 288 128 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R7 SYMATTR Value 10k TEXT -232 536 Left 0 !.tran 1 uic JF | |||||||||||||||||||||||||
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Posted by John Fields on May 10, 2008, 9:52 am
Please log in for more thread options On Sat, 10 May 2008 08:35:11 -0500, John Fields
>
>>Hi,
>> >>I have two common anode 7-segments displays, and only one wire to drive >>them. I need to achieve the following : when the control signal is +5V, >>display1 is ON and display2 is OFF. When the signal is 0V, display1 is >>OFF and display2 is ON. When the signal is not connected (high >>impedance), both displays are OFF. >> >>I tried using a NPN transistor for display1 and a PNP for display2, >>connecting their bases together. It works when the driving signal is >>present, but when the signal is floating current flows from the base of >>the PNP to the base of the NPN and both transistors turn each other on, >>resulting in both displays being ON. >> >>The circuit that didn't work : >> >> VCC >> + >> | >> | >> ___ |< >> o---------------------|___|--| >> | |\ >> | VCC | >> | + | >> | | Display2 >> | | | >> | ___ |/ | >> Input---o---|___|--| GND >> |> >> | >> | >> Display1 >> | >> | >> GND >> >> >> >>Is there a way to achieve this ? >
>--- >Yes. View in Courier: > > > >+5V>-------+-----------+-----------+-----------+
> | | | |
> [100k] [100K] | | > | | | E > +-----------|-----------|---------B 2N3906 > | | | C > | | E | > | +--[10K]--B 2N3906 | > [10k] | C | > | | | | > | C |A |A >VIN>-------+--[10k]--B 2N3904 [DISPLAY] [DISPLAY]
> E | |
> | [150R] [150R] > | | | >GND>-------------------+-----------+-----------+
--- Oops... Missed the part about when the input signal floats. JF | |||||||||||||||||||||||||
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