scmitt trigger and 0-gate

[labview1958]

I have connected the output of a schmitt trigger to the pin 24 DGND and the (Vee + GND) of the schmitt trigger to pin 48 0-Gate of a 6025E Daq card. The measure period fail to detect the period of the output of the schmitt trigger. However a test led connected to the output of the schmitt trigger lights up in pulses. Thus there is a square wave out of the trigger. A frequency generator connected to pin 24 and pin 48 gave a correct reading. What shall I do?

[Conseils]

By a 'schmitt trigger' you could be referring to over a hundred devices or even a custom circuit.

Post details of the devices, even better a clear circuit diagram....

As a guess for example, perhaps its an open collector output or you need to pop a series resistor in the output to make it compatible with the gate input? Perhaps there is a DC level on the 'schmitt trigger' output which is causing the input not to see the switching. An LED would not indicate this.

[labview1958]

I have attached my complete circuit. This problem has been bugging me for quite a while.

[labview1958]

I am using a 6V dc power supply. If I attached the LED to the output of the comparator it will pulse about once per second. The opto switch is measuring the rpm of a slow moving disc.

[labview1958]

The Rs 306 128 scmitt trigger is actually LM319. Is the circuit given sound?

[labview1958]

Anyone knows why the measure period.vi can measure the signal from the circuit shown in the attached file in the earlier post. Should I add some capacitors.

[johnsold]

One thing seems strange about your circuit. You have the output of the comparator connected to the DAQ ground and the common of the external circuit, includig the opto device connected to the signal input of the DAQ board. Ususally these would be reversed. If you are truly using a battery or a floating power supply, it will probably work. If you did this to get the polarity the way you want, it would be better to reverse the inputs to the comparator or to use the second comparator in the package as an inverter (if it is not being used for something else).

I did not do any calculations, but the values seem reasonable.

Lynn

[labview1958]

I am using the ordinary lab power supply connected to the mains. I am not using batteries. I have corrected the resistor values. Also I have reversed the DAQ cnnections concerned. At the moment I do not have any batteries. Pray, why should I use batteries or a floating point supply. Why?

[johnsold]

Well, I am not going to try to teach you electrical engineering in one post - the Forum has a 5000 character limit.

Basically, all circuits have to have a reference voltage, usually called ground or common (in the USA). Exceptions exist, of course, but most cases tie all the commons together. In your circuit everything connected to the bottom line should be connected to the DAQ digital ground. Your diagram used the symbol for a battery to represent the power source for the sensor and comparator circuit. Since that symbol is often used to generically to represent any DC power source, I was uncertain of your actual situation.

Batteries or floating power supplies can be used in situations where circuits do not have a convenient or safe means of tying the commons together. For example large motor drives may have currents flowing in the neutral conductor that are large enough that several volts may drop across the length of the conductor bewteen the motor and ground. This voltage could produce errors or damage the DAQ device or computer. An isolated power source like a battery could be used to power a sensor and allow the reference voltegs to be connected in such a manner as to avoid the risk of errors or damage.

Lynn

[labview1958]

I am mesuring low frequency between 0 and 200Hz. Can my circuit do that? Can it be improved? I have to use the one counter method. Can I change the 6V battery to the 5V output from the DAQ?