yes. The R1:R2 ratio must be large to keep current low through the igniter, but this will also lower the differential voltage into the op amp. You need to find a good ratio where you're amplifier can accurately measure the voltage but there is not a lot of current through the igniter.Originally posted by basil4j
1) Does the cont test op-amp bit, get its voltage reading from what is effectively a voltage divider with R1 being the 47K resistors I circled in the diagram, and R2 being the load/ignitors?
Yes, it allows the continuity tests to work while it's disarmed.2) Also, the safe/arm switch. What is the purpose of the 1k resistor? It looks as though it allows a constant, but limited, current to flow through for the cont test. It is obviously bypassed when the arm switch is armed.
it won't with those resistor values. this gets sort of complicated... for the input to go high, the voltage over that resistor (call it R1) will have to go above the Vh of the MCU inputs (obviously). This will be:3) Last question How would the safe/arm input going to the MCU ever go high?
If the arm switch is off, then the MCU input would be grounded by the 1k resistor, as the resistance to +9v is a fair bit higher. If the arm switch is on, then it would be a direct ground and hence no change.
Also, even if you want to eventually make the jump to 3 axes of acceleration, I don't recommend the 278. This is because it works out much nicer to use one Hi-g single axis sensor for your vertical direction and a second low G 2 axis sensor for the X and Y. Three axis sensors are very limited in variety, and I doubt one exists that fits this application. Stick with the ADXL78 for now, then adding on the other axes is a matter of adding a second cheap sensor instead of replacing it with a much more expensive (and less precise on the X and Y axes) sensor.
0.5mv < VL (load voltage must be greater than 0.5 millivolt to read continuity)
IL = 9 / (R2 + R1 + RL) > 0.5 ma
Correct.Originally posted by basil4j
Also, when you say 9(R1) in your equations, I assumed you meant 9R1? (9*R1)
When I wrote that, I was kind of assuming RL was one ohm in my head... so if you go by the parameter VL> 0.5 mv, then the equation should instead be:Where these statments refering to the same thing? I got a bit confused...
Originally posted by mtwieg
When I wrote that, I was kind of assuming RL was one ohm in my head... so if you go by the parameter VL> 0.5 mv, then the equation should instead be:
9*RL / (RL + R1 + R2) > 0.5 mv
Originally posted by mtwieg
Someone else threw me the idea of instead of using an op amp for each signal, one could use a single op amp with an analog multiplexer selecting its inputs. This would be efficient if you wanted to measure a lot of voltages with the same gain. So instead of maybe 6 six precision op amps each with their own resistors and each needing an input pin, you would just need one op amp with resistors plus the multiplexer and one input and three outputs (to select the input). But if it's only 3, it's probably not worth it. I'm not sure how an analog switch would handle such low voltages either, though many of them claim to operate rail to rail. They also get expensive. Something like the MAX4582ESE might be a good compromise.
Also, if the space of discrete components is getting big, consider resistor arrays. They are cheap as dirt (like 4 cents for a 4 X 0603 package) and tiny. They will not be precise, though, so you shouldn't use them to replace some resistors (like gain resistors).
Dont worry about the tracks under the component. As long as you dont mix up things like high speed databusses or switched power lanes with sensitive analog circuits, you shouldnt experience any problems. If I understand you correctly, you dont intend to measure the resistance of the igniter very precisely, so its even less critical. If youre interested in single opamps, with a smaller footprint, look out for devices in the SOT 23-5 (or -6) package. The SC-70 package is even smaller, but I guess it may be nasty to solder. Unfortunatly, you are looking for a very special OPA (low Vos, R2R Input, Vsupply >5V), so there is only a limited selection available. The best one, that I found is the LMP7701, but its input offset is higher (200µV). Therefore, the sensing current should be at least in the region of about 1mA.Originally posted by basil4j
I have been trying to use the OP747 (4 op-amp package) but am having difficulty with track layout as I am trying not to run many tracks under the component. Might give it another go though
A transistor is a simple semiconductor amplifier. It transforms a small input current (or voltage in case of a FET) into a bigger output current. If the input signal is increased, it acts more like a switch, because the output current is then limited by other parts of the circuit (if not, the transistor can be considered a mini smoke bomb ).Originally posted by ghost
This is sorta an off-topic question, but what does a transistor do? I understand a bit about electronics, but I still still don't get what a transistor is.
Thanks!
Good points. I think the inputs to the FETs would qualify as sensitive analog traces though, since any noise will effect the outputs significantly. However, you could get around this by just having the ADC sample and average a bunch several times to attenuate its effect. I wouldn't be too worried about interference with the continuity checkers. The sensor outputs, MOSFET gates, and power supplies should be your biggest concerns.Originally posted by Reinhard
Dont worry about the tracks under the component. As long as you dont mix up things like high speed databusses or switched power lanes with sensitive analog circuits, you shouldnt experience any problems. If I understand you correctly, you dont intend to measure the resistance of the igniter very precisely, so its even less critical. If youre interested in single opamps, with a smaller footprint, look out for devices in the SOT 23-5 (or -6) package. The SC-70 package is even smaller, but I guess it may be nasty to solder. Unfortunatly, you are looking for a very special OPA (low Vos, R2R Input, Vsupply >5V), so there is only a limited selection available. The best one, that I found is the LMP7701, but its input offset is higher (200µV). Therefore, the sensing current should be at least in the region of about 1mA.
Ugh, I remember when I first dived into electronics alone... transistors completely eluded me. There are so many ways to model them that I was constantly confused. I think I burned about two packs of radioshack transistors before realizing that it was a good idea to put limiting resistors on themA transistor is a simple semiconductor amplifier. It transforms a small input current (or voltage in case of a FET) into a bigger output current. If the input signal is increased, it acts more like a switch, because the output current is then limited by other parts of the circuit (if not, the transistor can be considered a mini smoke bomb ).
More information can be found in the Wikipedia:
https://en.wikipedia.org/wiki/Transistor
Reinhard
Originally posted by Reinhard
Dont worry about the tracks under the component. As long as you dont mix up things like high speed databusses or switched power lanes with sensitive analog circuits, you shouldnt experience any problems. If I understand you correctly, you dont intend to measure the resistance of the igniter very precisely, so its even less critical. If youre interested in single opamps, with a smaller footprint, look out for devices in the SOT 23-5 (or -6) package. The SC-70 package is even smaller, but I guess it may be nasty to solder. Unfortunatly, you are looking for a very special OPA (low Vos, R2R Input, Vsupply >5V), so there is only a limited selection available. The best one, that I found is the LMP7701, but its input offset is higher (200µV). Therefore, the sensing current should be at least in the region of about 1mA.
Originally posted by mtwieg
As for the op amps, the OP7xx series is really the only reasonably part I could find. A quad TSSOP package for <$6 with very low quiescent current and offset is about as good as it gets.
Originally posted by Reinhard
(if not, the transistor can be considered a mini smoke bomb ).
Originally posted by mtwieg
Good points. I think the inputs to the FETs would qualify as sensitive analog traces though, since any noise will effect the outputs significantly. However, you could get around this by just having the ADC sample and average a bunch several times to attenuate its effect. I wouldn't be too worried about interference with the continuity checkers. The sensor outputs, MOSFET gates, and power supplies should be your biggest concerns.
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