Triggring a 555 timer in monostable mode with a compartor output
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From: John Fields on 28 May 2010 11:06 On Fri, 28 May 2010 07:07:11 -0700 (PDT), chuck <chuckd87106(a)gmail.com> wrote: >I appreciate your input, > >I will try your suggestions and post the results. For now, I simply >buffered the input to the 555's trigger from the comparator, and it is >now working as it is supposed to. If it is a low-going transient on >the output of the comparator which is causing the problem, wouldn't it >die out on its own given enough time? --- Yes, but if it stays low while the 555 is powering up it'll cause an output pulse to occur.
From: Jamie on 28 May 2010 18:19 chuck wrote: > On May 27, 6:52 am, John Fields <jfie...(a)austininstruments.com> wrote: > >>On Wed, 26 May 2010 16:09:22 -0700 (PDT), chuck >> >> >> >><chuckd87...(a)gmail.com> wrote: >> >>>On May 24, 4:43 pm, Jamie >>><jamie_ka1lpa_not_valid_after_ka1l...(a)charter.net> wrote: >>> >>>> You could be experiencing a problem of the trigger getting an initial >>>>sink pulse on power up. >> >>>> To fix your problem, you could use a 100ohm R from the VCC to the RESET >>>>pin and a .1uf from the RESET pin to common.. This will force a short >>>>reset of the timer on initial power of Vcc. It will force it in the off >>>>state. >>>> You most likely have an issue with some capacitance that is causing a >>>>short SINK pulse on the trigger during initiation. You need to make sure >>>>the RESET input stays lower during the initiation state. >> >>>Thanks again for the advice, >> >>>I tried that, and unfortunately it didnt fix the problem. Still, when >>>the trigger input to the 555 comes from the output of the comparator >>>(or Vcc) the output of the 555 is high, but when I switch the trigger >>>input to Vcc directly, the output goes low (and stays that way untill >>>I decrease the input to 1/3 Vcc) >> >>--- >>Sound to me like what's happening is that there's a low-going >>transient from the comparator triggering the 555 on power up. >> >>Since the 555's RESET voltage is 1.2V max, at any supply voltage, then >>what you need to do is hold the reset pin lower than 1.2V for as long >>as it takes the transient to die away. >> >>Assuming a 5V supply, 100 ohms and 0.1�F gives a time constant of >>2.7�s from 0V to 1.2V, (the 555's RESET release voltage) which is way >>too fast if what you're trying to do is keep a turn-on transient from >>the comparator from triggering the 555. >> >>Here's a circuit for a 555 one-shot with a passive power-on reset: >>(View in Courier) >> >>.Vcc>-+---------+-----+-------+--------+----+ >>. |R1 |R2 |R3 8| |R4 | >>. [10k] [10k][910k] +---+---+ [100k] | >>.__ | C1 | Rt| 2|_ Vcc _|4 | |C3 >>.IN>--+-[100nF]-+-----|--O|T R|O---+ [100nF] >>. | 6| | |C4 | >>. +---|TH 555| [100nF] | >>. | 7|_ |3 | | >>. +--O|D OUT|O---|----|--->OUT >>. +| | GND | | | >>. [1�F] +---+---+ | | >>. Ct|C2 1| U1 | | >>.GND>-----------------+-------+--------+----+ >> >>What we have to do is figure out the values of R4 and C4 which will >>keep U1-4 below 1.2V for the time required to let the transient on >>U1-3 die away. >> >>Just for grins, lets say 10ms will do it and were running a 12V >>supply. >> >>Then we can write: >> >> Vcc >> t = RC ln ----------- = KRC >> Vcc - Vth >> >>Where T is the time required, in seconds, >> R is the resistance required, in ohms, >> C is the capacitance required, in farads, >> Vcc is the supply voltage in Volts, >> Vth is the threshold voltage, in Volts, and >> ln is log to the base e. >> >>if we solve for: >> >> Vcc 12V >> K = ln ----------- = ln ------------ = 0.105 >> Vcc - Vth 12V - 1.2v >> >>then we can simplify to: >> >> t = KRC >> >>and solve for either R or C, knowing the other. >> >>Let's say that we arbitrarily choose 1 megohm for R; then we can >>rearrange and solve for C: >> >> T 1e-2s >> C = ---- = ---------------- = 9.52e-8F ~ 10nF = 0.01 microfarads. >> KR 1.05e-1 * 1e6R >> >>So now you have: >> >>.Vcc>-+---------+-----+-------+--------+----+ >>. |R1 |R2 |R3 8| |R4 | >>. [10k] [10k][910k] +---+---+ [1M] | >>.__ | C1 | Rt| 2|_ Vcc _|4 | |C3 >>.IN>--+-[100nF]-+-----|--O|T R|O---+ [100nF] >>. | 6| | |C4 | >>. +---|TH 555| [10nF] | >>. | 7|_ |3 | | >>. +--O|D OUT|O---|----|--->OUT >>. +| | GND | | | >>. [1�F] +---+---+ | | >>. Ct|C2 1| U1 | | >>.GND>-----------------+-------+--------+----+ >> >>If you need more time you can make either R4 or C4 bigger, and C3 is a >>_must_ if you're using a bipolar 555 and should be connected directly >>across pins 1 and 8. If you're using a CMOS 555 then it isn't needed. >> >>JF > > > I appreciate your input, > > I will try your suggestions and post the results. For now, I simply > buffered the input to the 555's trigger from the comparator, and it is > now working as it is supposed to. If it is a low-going transient on > the output of the comparator which is causing the problem, wouldn't it > die out on its own given enough time? > > Thanks, > Chuck Yes it would how ever, if it takes longer to dissipate than the timer RESET circuit event last, you'll still have your problem.. The timer RESET must last longer than it does to get the trigger level above 1/3 of VCC.
From: chuck on 29 May 2010 02:15 On May 28, 4:19 pm, Jamie <jamie_ka1lpa_not_valid_after_ka1l...(a)charter.net> wrote: > chuck wrote: > > On May 27, 6:52 am, John Fields <jfie...(a)austininstruments.com> wrote: > > >>On Wed, 26 May 2010 16:09:22 -0700 (PDT), chuck > > >><chuckd87...(a)gmail.com> wrote: > > >>>On May 24, 4:43 pm, Jamie > >>><jamie_ka1lpa_not_valid_after_ka1l...(a)charter.net> wrote: > > >>>> You could be experiencing a problem of the trigger getting an initial > >>>>sink pulse on power up. > > >>>> To fix your problem, you could use a 100ohm R from the VCC to the RESET > >>>>pin and a .1uf from the RESET pin to common.. This will force a short > >>>>reset of the timer on initial power of Vcc. It will force it in the off > >>>>state. > >>>> You most likely have an issue with some capacitance that is causing a > >>>>short SINK pulse on the trigger during initiation. You need to make sure > >>>>the RESET input stays lower during the initiation state. > > >>>Thanks again for the advice, > > >>>I tried that, and unfortunately it didnt fix the problem. Still, when > >>>the trigger input to the 555 comes from the output of the comparator > >>>(or Vcc) the output of the 555 is high, but when I switch the trigger > >>>input to Vcc directly, the output goes low (and stays that way untill > >>>I decrease the input to 1/3 Vcc) > > >>--- > >>Sound to me like what's happening is that there's a low-going > >>transient from the comparator triggering the 555 on power up. > > >>Since the 555's RESET voltage is 1.2V max, at any supply voltage, then > >>what you need to do is hold the reset pin lower than 1.2V for as long > >>as it takes the transient to die away. > > >>Assuming a 5V supply, 100 ohms and 0.1µF gives a time constant of > >>2.7µs from 0V to 1.2V, (the 555's RESET release voltage) which is way > >>too fast if what you're trying to do is keep a turn-on transient from > >>the comparator from triggering the 555. > > >>Here's a circuit for a 555 one-shot with a passive power-on reset: > >>(View in Courier) > > >>.Vcc>-+---------+-----+-------+--------+----+ > >>. |R1 |R2 |R3 8| |R4 | > >>. [10k] [10k][910k] +---+---+ [100k] | > >>.__ | C1 | Rt| 2|_ Vcc _|4 | |C3 > >>.IN>--+-[100nF]-+-----|--O|T R|O---+ [100nF] > >>. | 6| | |C4 | > >>. +---|TH 555| [100nF] | > >>. | 7|_ |3 | | > >>. +--O|D OUT|O---|----|--->OUT > >>. +| | GND | | | > >>. [1µF] +---+---+ | | > >>. Ct|C2 1| U1 | | > >>.GND>-----------------+-------+--------+----+ > > >>What we have to do is figure out the values of R4 and C4 which will > >>keep U1-4 below 1.2V for the time required to let the transient on > >>U1-3 die away. > > >>Just for grins, lets say 10ms will do it and were running a 12V > >>supply. > > >>Then we can write: > > >> Vcc > >> t = RC ln ----------- = KRC > >> Vcc - Vth > > >>Where T is the time required, in seconds, > >> R is the resistance required, in ohms, > >> C is the capacitance required, in farads, > >> Vcc is the supply voltage in Volts, > >> Vth is the threshold voltage, in Volts, and > >> ln is log to the base e. > > >>if we solve for: > > >> Vcc 12V > >> K = ln ----------- = ln ------------ = 0.105 > >> Vcc - Vth 12V - 1.2v > > >>then we can simplify to: > > >> t = KRC > > >>and solve for either R or C, knowing the other. > > >>Let's say that we arbitrarily choose 1 megohm for R; then we can > >>rearrange and solve for C: > > >> T 1e-2s > >> C = ---- = ---------------- = 9.52e-8F ~ 10nF = 0.01 microfarads. > >> KR 1.05e-1 * 1e6R > > >>So now you have: > > >>.Vcc>-+---------+-----+-------+--------+----+ > >>. |R1 |R2 |R3 8| |R4 | > >>. [10k] [10k][910k] +---+---+ [1M] | > >>.__ | C1 | Rt| 2|_ Vcc _|4 | |C3 > >>.IN>--+-[100nF]-+-----|--O|T R|O---+ [100nF] > >>. | 6| | |C4 | > >>. +---|TH 555| [10nF] | > >>. | 7|_ |3 | | > >>. +--O|D OUT|O---|----|--->OUT > >>. +| | GND | | | > >>. [1µF] +---+---+ | | > >>. Ct|C2 1| U1 | | > >>.GND>-----------------+-------+--------+----+ > > >>If you need more time you can make either R4 or C4 bigger, and C3 is a > >>_must_ if you're using a bipolar 555 and should be connected directly > >>across pins 1 and 8. If you're using a CMOS 555 then it isn't needed.. > > >>JF > > > I appreciate your input, > > > I will try your suggestions and post the results. For now, I simply > > buffered the input to the 555's trigger from the comparator, and it is > > now working as it is supposed to. If it is a low-going transient on > > the output of the comparator which is causing the problem, wouldn't it > > die out on its own given enough time? > > > Thanks, > > Chuck > > Yes it would how ever, if it takes longer to dissipate than the timer > RESET circuit event last, you'll still have your problem.. > > The timer RESET must last longer than it does to get the trigger > level above 1/3 of VCC.- Hide quoted text - > > - Show quoted text - Thanks to both of you for the help, I learned a valuable lesson on the importance of using de-coupling capacitors. Things worked just fine as soon as I put the 100nF cap between pin 1 and pin 8. Should I get in the habit of always using them on the Vcc pin on ICs?
From: krw on 29 May 2010 10:15
On Fri, 28 May 2010 23:15:54 -0700 (PDT), chuck <chuckd87106(a)gmail.com> wrote: >On May 28, 4:19�pm, Jamie ><jamie_ka1lpa_not_valid_after_ka1l...(a)charter.net> wrote: >> chuck wrote: >> > On May 27, 6:52 am, John Fields <jfie...(a)austininstruments.com> wrote: >> >> >>On Wed, 26 May 2010 16:09:22 -0700 (PDT), chuck >> >> >><chuckd87...(a)gmail.com> wrote: >> >> >>>On May 24, 4:43 pm, Jamie >> >>><jamie_ka1lpa_not_valid_after_ka1l...(a)charter.net> wrote: >> >> >>>> � You could be experiencing a problem of the trigger getting an initial >> >>>>sink pulse on power up. >> >> >>>> �To fix your problem, you could use a 100ohm R from the VCC to the RESET >> >>>>pin and a .1uf from the RESET pin to common.. �This will force a short >> >>>>reset of the timer on initial power of Vcc. It will force it in the off >> >>>>state. >> >>>> � You most likely have an issue with some capacitance that is causing a >> >>>>short SINK pulse on the trigger during initiation. You need to make sure >> >>>>the RESET input stays lower during the initiation state. >> >> >>>Thanks again for the advice, >> >> >>>I tried that, and unfortunately it didnt fix the problem. Still, when >> >>>the trigger input to the 555 comes from the output of the comparator >> >>>(or Vcc) the output of the 555 is high, but when I switch the trigger >> >>>input to Vcc directly, the output goes low (and stays that way untill >> >>>I decrease the input to 1/3 Vcc) >> >> >>--- >> >>Sound to me like what's happening is that there's a low-going >> >>transient from the comparator triggering the 555 on power up. >> >> >>Since the 555's RESET voltage is 1.2V max, at any supply voltage, then >> >>what you need to do is hold the reset pin lower than 1.2V for as long >> >>as it takes the transient to die away. >> >> >>Assuming a 5V supply, 100 ohms and 0.1�F gives a time constant of >> >>2.7�s from 0V to 1.2V, (the 555's RESET release voltage) which is way >> >>too fast if what you're trying to do is keep a turn-on transient from >> >>the comparator from triggering the 555. >> >> >>Here's a circuit for a 555 one-shot with a passive power-on reset: >> >>(View in Courier) >> >> >>.Vcc>-+---------+-----+-------+--------+----+ >> >>. � � |R1 � � � |R2 � |R3 � �8| � � � �|R4 �| >> >>. � [10k] � � [10k][910k] +---+---+ [100k] �| >> >>.__ � | � C1 � �| � Rt| �2|_ Vcc _|4 � | � �|C3 >> >>.IN>--+-[100nF]-+-----|--O|T � � R|O---+ [100nF] >> >>. � � � � � � � � � � | �6| � � � | � �|C4 �| >> >>. � � � � � � � � � � +---|TH �555| [100nF] | >> >>. � � � � � � � � � � | �7|_ � � �|3 � | � �| >> >>. � � � � � � � � � � +--O|D � OUT|O---|----|--->OUT >> >>. � � � � � � � � � �+| � | �GND �| � �| � �| � � >> >>. � � � � � � � � � [1�F] +---+---+ � �| � �| � � � � >> >>. � � � � � � � � � Ct|C2 � �1| U1 � � | � �| � � >> >>.GND>-----------------+-------+--------+----+ >> >> >>What we have to do is figure out the values of R4 and C4 which will >> >>keep U1-4 below 1.2V for the time required to let the transient on >> >>U1-3 die away. >> >> >>Just for grins, lets say 10ms will do it and were running a 12V >> >>supply. >> >> >>Then we can write: >> >> >> � � � � � � � � � Vcc >> >> � � t = RC ln ----------- = KRC >> >> � � � � � � � �Vcc - Vth >> >> >>Where T is the time required, in seconds, >> >> � � �R is the resistance required, in ohms, >> >> � � �C is the capacitance required, in farads, >> >> � � �Vcc is the supply voltage in Volts, >> >> � � �Vth is the threshold voltage, in Volts, and >> >> � � �ln �is log to the base e. >> >> >>if we solve for: >> >> >> � � � � � � � Vcc � � � � � � �12V >> >> � �K = ln ----------- = ln ------------ = 0.105 � >> >> � � � � � �Vcc - Vth � � � �12V - 1.2v >> >> >>then we can simplify to: >> >> >> � �t = KRC >> >> >>and solve for either R or C, knowing the other. >> >> >>Let's say that we arbitrarily choose 1 megohm for R; then we can >> >>rearrange and solve for C: >> >> >> � � � � � T � � � � 1e-2s >> >> � � C = ---- = ---------------- = 9.52e-8F ~ 10nF = 0.01 microfarads. >> >> � � � � �KR � � 1.05e-1 * 1e6R >> >> >>So now you have: >> >> >>.Vcc>-+---------+-----+-------+--------+----+ >> >>. � � |R1 � � � |R2 � |R3 � �8| � � � �|R4 �| >> >>. � [10k] � � [10k][910k] +---+---+ �[1M] � | >> >>.__ � | � C1 � �| � Rt| �2|_ Vcc _|4 � | � �|C3 >> >>.IN>--+-[100nF]-+-----|--O|T � � R|O---+ [100nF] >> >>. � � � � � � � � � � | �6| � � � | � �|C4 �| >> >>. � � � � � � � � � � +---|TH �555| �[10nF] | >> >>. � � � � � � � � � � | �7|_ � � �|3 � | � �| >> >>. � � � � � � � � � � +--O|D � OUT|O---|----|--->OUT >> >>. � � � � � � � � � �+| � | �GND �| � �| � �| � � >> >>. � � � � � � � � � [1�F] +---+---+ � �| � �| � � � � >> >>. � � � � � � � � � Ct|C2 � �1| U1 � � | � �| � � >> >>.GND>-----------------+-------+--------+----+ >> >> >>If you need more time you can make either R4 or C4 bigger, and C3 is a >> >>_must_ if you're using a bipolar 555 and should be connected directly >> >>across pins 1 and 8. �If you're using a CMOS 555 then it isn't needed. >> >> >>JF >> >> > I appreciate your input, >> >> > I will try your suggestions and post the results. For now, I simply >> > buffered the input to the 555's trigger from the comparator, and it is >> > now working as it is supposed to. If it is a low-going transient on >> > the output of the comparator which is causing the problem, wouldn't it >> > die out on its own given enough time? >> >> > Thanks, >> > Chuck >> >> Yes it would how ever, if it takes longer to dissipate than the timer >> RESET circuit event last, you'll still have your problem.. >> >> � The timer RESET must last longer than it does to get the trigger >> level above 1/3 of VCC.- Hide quoted text - >> >> - Show quoted text - > >Thanks to both of you for the help, > >I learned a valuable lesson on the importance of using de-coupling >capacitors. Things worked just fine as soon as I put the 100nF cap >between pin 1 and pin 8. Should I get in the habit of always using >them on the Vcc pin on ICs? Unless Vcc and Gnd are both on planes, yes. If they're on planes, and wiring to those planes is *short*, you can get away without decoupling caps on each part. You should have some larger caps somewhere on the board, too. |