Tech Forum Questions

If you want to know if the charging circuit is working, note the voltage with the switch on, motor off, then start the motor; the voltage should go up about 1 volt. If it goes into the red, the regulator is defective.

I suspect that your truck already has the circuit installed. If it is less than 20 years old it almost certainly does. It is called the "battery indicator light". That light does exactly what you want, except it is dark when you want an illuminated  green LED.

Put the keys in the ignition and switch to the ON position without starting the engine. All the warning lights on the dash should light up (this is a bulb check routine to identify burned out indicator bulbs or LEDs). After a couple seconds most of the warning lights will go off, except probably 2. One is the oil pressure warning light. The engine isn't running, so the oil pressure is zero, so the light is on. The other light still on will be the battery light. The engine isn't running so the battery is supplying all the power to the truck’s electrics.

Now start the engine. Those two lights should go off. The battery light is off which means the alternator is supplying all the power to the trucks electrical systems. That little indicator light basically just looks at the electrical system's voltage. A good, fully charged car lead acid battery will read 12.6V without any load. The charging system of a car runs at about 13.5V to 14V.

You can check the voltages with a voltmeter at the battery posts with the engine on and off. If the electrical system is at a voltage of 12.6V or less than the battery is carrying the electrical load. If the electrical system is at more than about 13V then the alternator is supplying all the electrical power.

If the battery needs charging then the alternator is doing so, as long as the battery indicator light is off. If the battery is not charging because it doesn’t need to be, then the light is also off. What the battery light won’t tell is the charge/discharge rate, which you would need an ammeter for.

The input to the circuit has an RC filter so the water valve doesn't turn on immediately when the switch closes.  When the water level falls and the switch on time exceeds the off time, the opamp will turn on Q1 which powers the solonoid valve.  As the water rises and the switch off time exceed the on time, Q1 turns off.

You asked for a circuit that would provide about 15 to 30 seconds delay before sending 12 volts to your water relay, because the float switch bounces which keeps the water constantly turning on and off. The on delay timer circuit in figure 1 will give you that delay.

25 to 30 seconds after the float switch closes (and stops bouncing), the output on pin 3 of the 555 will go positive and the MJE3055 NPN power transistor will conduct to turn on the existing water relay. The 555 will keep the MJE3055 conducting until the float switch opens.

However, I think there is a better approach to your problem. Even with the delay you wanted, the water will still turn on and off constantly as the water level raises and lowers at the set point, closing and opening the float switch. It is better to use two level switches, set to transfer at an upper limit to turn the water off and a lower limit switch set to turn the water on. When the lower switch transfers, water will flow and continue until the upper switch transfers. The float switch circuit in figure 2 will do that. The vertical distance between the switches will determine how often the water runs and how long it takes to raise the water in the pool from the low level to the high.

I solved this problem many times over the years by using 2 floats in a simple start/stop pushbutton configuration. This comes from the common push button controls used by major motor control systems. One float needs a N.C. contact and the other needs a N.O. contact. Many floats have both. In your application, the top level float should operate N.C. contacts as the Stop button. The lower level float, which starts the pump, would use the N.O. contacts as the Start button.

The power relay needs an extra N.O. contact to serve as a holding contact and is wired in parallel with the N.O. contact of the Start float.

In operation, with a dropping water level, the Stop button contacts would be closed. As the water level continues to fall, the Start float N.O. contact would close and start the pump. The holding contact in parallel then holds the motor drive relay in the energized condition with no regard for any bouncing by the Start float. The pump would continue to run until the level rose sufficiently to trip the Stop float and stop the pump motor. Falling water level will then reset the Stop float.

The water level will continue to fall again until the Start float again closes and the cycle repeats. Run time of the pump is determined by the vertical distance between the Stop and Start floats and the flow rate of the pump.

The easiest solution is to use a time delay relay. If the relay is on for 30 seconds straight, then the contact will close for the relay. So wire the float switch in series with the time delay relay COIL. Then wire power thru the relay contact to the water valve. So the water valve will come on when the float is low for 30 continuous seconds - until the float switch opens up. They have a 30 sec and a 60 sec time delay relay with socket at www.mpja.com for $9.95. You should probly put a reverse diode across the time delay relay - so you don't burn out the contacts on the water float switch. If you need a schematic - email me at jimsATruralhostDOTcom and I should be able to come up with one for ya......

Debouncing might fix the problem. Adding hysteresis into the circuit might also work. But you might consider fixing the problem the way the problem of float switches in a fluid is usually fixed: use two switches (which both debounces and adds hysteresis). A lower float switch turns on the water supply when the level falls below the lower limit. The supply then runs until the water level rises above the upper float switch, where it turns off. Look up the circuitry used for septic tank and sump pump controllers that use two float switches.

The pool-filler setup can use a low-pass filter, but not the electronic kind.

Take a piece of large-diameter PVC pipe (see figure) and glue a cap on one end (the bottom). Drill a small hole near the bottom below the level at which you want to start the pump. Drill a large hole above the level at which the the pump or valve should stop the flow of water.

This arrangement provides an aquatic low-pass filter. The small hole does not let waves or disturbances in the pool affect the level in the tube. The large failsafe hole lets water get into the pipe quickly in case the the small hole gets plugged and cannot let water fill the tube.

So if the water reaches the failsafe hole it immediately floods the tube and causes the level switch to turn off immediately. Think of the small hole as a high resistance connected in series with a capacitor — the large tube. Attach the tube to the side of the pool with a bracket or a pair of suction cups.

Gary has described what is known as a "switch debouncer." The MC14490 contains six switch debounce circuits which may be cascaded in order to create a longer time delay than a single one will provide. The IC also contains an oscillator circuit whose frequency is controlled via a single external capacitor.  This IC followed by a transistor can be used to power his relay.

From your description it looks like the wave motion is exceeding the movement differential or differential gap of your switch. I have a couple of solutions.

#1 Mechanical Solution (easy):  Insert the float in a piece of PVC pipe which is large enough to allow free movement of the float. In industrial applications this is called a stilling well and it works by shielding the float form the wave action of the pool surface.

#2 Electrical Solution (more complicated): By using two Normally Closed (NC) float operated switches and a 12V relay with an extra set of NO contacts, you can build a circuit to turn the solenoid valve on at a "low" level and off at a "high" level. Wire the relay coil, Low float switch and High float switch in series between the +12V Source and Ground. Wire the "extra" relay contact in parallel with the Low float switch and the other relay contact in series with the solenoid valve. This arrangement works as follows:

(a) The two float switches are Open when the float is high, when the level drops to the "Low" level the Low float switch closes and activates the relay which closes the relay contacts.

(b) as water runs into the pool the "Low" float switch opens up but the "Hold In" contact on the relay keeps the relay (and thus the solenoid valve) activated allowing the pool to continue filling.

(c) When the pool level reaches the "High" float, the High float switch activates (opens) and the relay and solenoid valve deactivate stopping the flow of water. You can experiment with the "low level" position to obtain the operation you want.

CAUTION: Water and electricity do not mix. Any electrical devices used around water where humans can contact the electrified water should be fed through a ground fault interrupt device.

An effective method to prevent short-term fluctuations from exercising the float switch is to dampen the frequent rise and fall of the water level by a non-electronic method. Just enclose the float switch in a vessel (such as a one-gallon milk container with the top cut off) and make a very small hole in the bottom of the container. This is the equivalent of adding a large capacitor to a varying dc voltage to smooth out the variations. The float switch will respond to an average level over time. Depending on the size of hole, it will take some time for the water level in the container to rise or fall. I believe this is essentially what lake level monitors use to tune out temporary wave action.

You didn't say, but I'll presume you have the classic, small (8"x6"x4") model 7-100 that I have. Mine sits for long periods unused and always is scratchy.  But, so far, it always cleans up by rotating the control shaft through its entire range for a minute or so. I usually do this before I turn it on. If that doesn't do it, cleaning is the next step. DeoxIT D5 is a product most used now in the amp repair business. I think RadioShack even carries it. The pot on mine is sealed in the normal opening at the terminals, but I saw a small hole in the side or you might be able to get some cleaner around the back side of the shaft. I don't think I have ever cleaned mine.

I saw  a couple of schematics floating around at Activity Stream - Music Electronics Forum.  But you don't need one to replace the pot. Mine is 50K ohm, but I also saw 10K ohm on a schematic. There are several versions of this amp, updated over the years. I presume these are logarithmic/audio taper. You can check the value and taper with an ohmmeter. The pot is really small and has the power switch included. I haven't seen any replacement parts listed or mentioned. One of the best and cheapest parts sources is Mouser Electronics - Electronic Components Distributor. You would have to remove the battery holders and metal shield to use any larger pot. You could get by without the power switch, since power is removed when the cable is removed from the input jack, like is done on many guitar FX pedals.

You can SAFELY clean a pot with a spray cleaner like "Deoxit D5" (item #64-249 @ RadioShack). Just spray a small amount of it into the pot (typically in the case opening where the solder tabs are), rotate the pot through its' range a few times, then repeat once or twice more.  This should clean up your pot nicely.  NOTE - This stuff is also good for cleaning noisy/intermittent switches (make sure there's NO POWER APPLIED to the switch!).

However, if it doesn't help (a possibility, depending on the age of the pot and its overall use), you can replace it with a similar unit. Just unsolder ONE LEAD from one of the end tabs and use a DMM to measure the resistance between the end tabs - this will give you the resistance value of the pot. In many cases, your local RadioShack will have a similar-value pot in stock - if not, get the next larger value pot that's closest to what you read (i.e., your pot read 7.5K, the next closest value would be 10K). If The Shack doesn't have anything close, order one through DigiKey (www.digikey.com) or Jameco (www.jameco.com).  IMPORTANT: make sure you replace it with an audio taper pot, not a linear taper pot.

RadioShack may still sell an aerosol can of potentiometer/contact cleaner, just use it sparingly!

If the sounds still cuts out at certain spots, the control is probably bad and needs to be replaced. If you can use a soldering iron this is a simple job.

Connect an ohmmeter across the two outer tabs, you will not get a reading if the control is broken. Now take a reading from the center tab to each of the outer tabs and add them together to get the value of the new one to buy.

The best fix is to replace the pot. If you don't want to do that, then a spray on cleaner is available at most RadioShack or electronic parts supplies that will supposedly clean these pots. I've never had much success with the spray stuff though. It's supposed to wash away the carbon dust and leave a protective slippery film behind to prevent further ware. My preferred method is to disassemble the pot, clean it thoroughly with alcohol, re-lube it with pot grease, then reassemble it. Even all this doesn't permanently fix the problem, it will return.

The problem is usually caused by the carbon resistance material breaking down. Most pots have their value stamped somewhere on the case, generic replacements are available and are  cheap and easy to install.

When choosing a wall wart you have to check three things and look out for a problem with wall warts.

1. Does the voltage match?
2. Does the polarity match?
3. Does the walwart put out enough current?

The catch is with the voltage. Wall warts come in four different flavors.

1. Transformer-unregulated.
2. Transformer-regulated.
3. Switching-unregulated (very rare).
4. Switching-regulated.

The belief that the general public has about power supplies is that the supply will drive whatever current it is rated at.

This is WRONG in capitol letters. The device will draw whatever current it is rated at at the rated voltage. The power supply must be able to supply at least as much current as the device needs. If it can't the voltage will fall off. If it is rated at more current than necessary this won't hurt anything as long as the maximum voltage is not exceeded, and this is the catch.

This is why it’s important to check unregulated wall warts at their rated current. A 9 volt unregulated power supply may read as high as 18 volts with no load on it. So if an unregulated supply is rated at 1 amp at 9 volts and the device only needs 250mA or 1/4 amp the voltage output of the supply could be 12, 14, or even 18 volts. OOPS, here comes the smoke.

A regulated wall wart will put out the same voltage plus or minus some percentage loaded or unloaded. A switching wall wart will be very light compared to a transformer walwart of the same amperage rating.

Yes, the tip needs to be positive.

The symbol you see on the radio indicates a "coaxial" power adapter plug with "tip positive" polarity. Go to your local RadioShack and get a "Universal" wall wart that will deliver 9VDC @ 1500 (or more) mA DC (IMPORTANT: Take your radio to the store so you can get the proper "adaptaplug" to use with the wall wart!).

I suggest a 1500 mA (i.e., 1.5A) or larger current-rating to ensure you'll have enough power. DO NOT FEAR: a wart with a large current rating WILL NOT harm the radio - however, a smaller-than-needed current rating will quickly burn out the wall wart (or cause overheating and/or fire hazard).

Hope this helps and happy listening to your radio.

You are on the right track!

It will not harm your circuit to use a Wall Wart with a higher current (milliamp) rating. This is just like using a bigger battery; think D-cell vs. AA.

A Wall Wart with a lower current rating is like using  weak battery. You may experience some distortion, lower volume, etc. This is not harmful to the circuit.

Just remember to use the correct voltage wall wart, this you cannot play around with!

As long as you use a 9 volt center pin positive adapter, you will be fine. Current will be limited by the resistance of the radio. There may be a problem if the current supplied by the wall wart is too low, as this may cause the voltage to drop. Using a 1 or 2 amp wall wart should be OK, and if the current of the wart is too high, it won't damage your radio (as long as the output is 9 volts).

OBTW, I am assuming this is a transistor radio, and not a real old "vintage" tube radio.

The problem with LEDs is that their operating voltage is way below the 120VAC in the home. So you either string a whole bunch of LEDs in series to get up to the 120VAC (which is 165V peak) drop, or you put a resistor in the circuit (and waste tons of power as heat), or you need a step down voltage converter. It is the step down voltage converter that has problems with a typical dimmer circuit. The common triac based dimmer gives a badly distorted AC waveform that wreaks havoc on inductor based step down voltage regulators.

LED light bulbs are dimmable just like incandescent bulbs.

Fluorescent bulbs require a minimum voltage so as to be able to ionize the mercury vapor in the bulb. Without the ionization no current can flow and the bulb won't generate any UV light. It is the UV light that causes the coating of the bulb to fluoresce. So a special circuit is required to make a dimmable fluorescent bulb.

An LED will start to output light as soon as the minimum avalanche voltage is exceeded. The amount of light that is output is then determined by the current flow. The more current the brighter the light output. The current flow determines how hot the junction gets. When you exceed the maximum rated temperature the junction will melt and the LED will fail.

An incandescent bulb is a positive coefficient resistor. The hotter the filament the higher the resistance so the current is self limiting. You have to increase the voltage to increase the current flow and to increase the light output. When you exceed the maximum rated voltage of the bulb the filament will melt and the bulb will fail.

Have you ever noticed that most incandescent bulbs fail when you turn them on? That is because of the inrush current which overheats and melts the filament at the weak point. Sometimes you can fix an incandescent bulb for a while by lightly bumping the side of the bulb. The filament will weld back together, but it is always a weak joint that will fail again in a very short time.

So just like incandescent bulbs, LED bulbs should be dimmable whether or not the package says so. I may be wrong and I am going by my training and knowledge and not by empirical testing.

Besides solid state relays you can just use an analog mux/demux chip (that'll cost about $1). The mux/demux will take 8 inputs and, based on the address pins, route one of those inputs to the output. (Or, you can run it backwards and take one input and route it to one of eight outputs). Something like a MC74HC4051 or HEF4051B.

The challenge may be the fairly low voltage and current from the thermocouples, aggravated by long signal lines. You might need to put amplifiers at the thermocouples. Or, for a couple bucks, replace the thermocouples with sensors much better designed for remote sensing over narrow temperature ranges near room temp. Something like LM34DZ.

First, people have a misconception that the junction of dissimilar materials form a thermocouple. Actually, the junction creates a return path for current that flows due to the Seebeck effect, discovered in 1822. In essence, a temperature difference between the ends of a conductor creates a small EMF, or voltage, between the ends. A type-E thermocouple (constantan and chromel wires) has a 68 µV/°C coefficient, which means you'll see a 680 µV increase for a 10°C increase in temperature between the ends of the thermocouple wires. Of course, you need to know the temperature at the switches you use in your measurement systems. People often refer to this as cold-junction compensation, and many diagrams show an ice bath at 0°C. You just need an accurate temp sensor, not an ice bath.

Second, I recommend reed relays; either two SPST or one DPST relay will do the job nicely. Some people might say, "Wait, all the connections to the relays create individual thermocouples." Not so, the difference in voltage between the ends of the relay conductors, and associated conductors, amounts to a negligible amount. Just keep your equipment at a fairly stable temperature. Depending on the ADC you use, you might need an accurate instrumentation amplifier between the thermocouple signal from the relays to the ADC. Again, the measured voltage results from the temperature difference between the end of the thermocouple and the temperature measured at thermocouple wire ends in your measurement system.

For more information, I recommend the free ebooks, "Switching Handbook" and "Low Level Measurements Handbook," from Keithley Instruments: [url=http://www.keithley.com/knowledgecenter]http://www.keithley.com/knowledgecenter[/url]. Also, see the article at: [url=http://tinyurl.com/kuvd93o]http://tinyurl.com/kuvd93o[/url].

You can use a solid state relay. They are available in DIP packages of up to 8 switches (I believe) and can be controlled by your microprocessor.

It sounds like you have a CO2 gas laser... whether it's a gas or solid state laser, the very best resource by far for learning about and designing a power supply for your laser is Sam's Laserfaq - www.repairfaq.org/sam/lasertoc.htm.

I have been interested in lasers as a hobby for years and when I first became interested this was my goto site for a long time. Of course there are many websites covering lasers or laser shows (my main interest) whether hobby or commercial, but this site is where the deep knowledge of lasers (as well as other electronics topics) has been deposited.

One thing you MUST be familiar with is laser safety, especially operating a 50 watt invisible wavelength laser. The laser FAQ has plenty of information along those lines also. You will find that early in the table of contents. Best of luck to you in your search for a suitable power supply, and be very careful - protect your eyes before you power it up!

Try the circuit used here www.die4laser.com/dvd-rec/Die4Drive.htm as a starting point. Obviously you will need to size the MOSFET and heatsink appropriately as well as the current sense resistor for the power involved, but it should get you going in the right direction. A standard ATX computer power supply can often be repurposed as a high current DC source to power the thing.