Hi all, I am looking to add a motion sensing circuit to a CRT clock that I recently built. I would like the sensor disconnect one of the cathode ray tube's heater wires when motion is no longer detected after 10-15 minutes in order to prolong the life of the tube. (the CRT heater is 6.3v.). I am thinking a PIR module and 555 timer that would trigger a relay to "open" the heater wire. I have 12v. dc. going to a part of the clock that I can tap into, so I would probably also need to use a voltage regulator in the circuit. I am pretty adept at reading schematics and soldering, but don't have the knowledge to put all this together in a workable circuit. Does anyone have a similar schematic? Thanks for any help!

 

Welcome to AAC.

I've done a few PIR sensors on the forum. Take a look at post 19 on this thread.

You can ignore the low battery indicator and latching alarm circuit. Essentially, you just connect pin 3 from the 555 to a relay coil (with a protection diode) which then controls the CRT. You can use 12VDC to drive the circuit as long as you use a regulator or Zener diode to limit the voltage to the PIR module to 5-6VDC.

The circuit shown in post 19 is designed to ignore the PIR sensor once tripped until a) the timing is complete and b) the PIR goes off then on (detects no motion, then detects again). If you want the PIR to keep the CRT on irregardless, then you can leave off C3 and R8.

Swap C6 for 1000uF or VR1 for 10MΩ to get about 1000 seconds or up to 16.5 minutes.

If this looks ideal, I can redraw the circuit to meet your needs.

 

Thanks elec_mech, that circuit looks like it should work fine. I would not need the low bat. or latching circuits. (not sure what latching is). I think swapping vr1 would be the best way to go. If you could draw me a circuit with the voltage regulator, relay, protection diode, and anything else I may need I would be most grateful! What would be a good relay to use? I may just be able to bring this 18 month project to a happy end! Thanks again!

 

A latching circuit is one that stays on after the trigger signal is removed. Think of turning on your computer - you press the power switch once then release it. The switch is a momentary one, so it disconnects once your finger is removed, but the computer comes on and stays on due to a latching circuit of sorts.

Okay, I think I got it. Finding 10MΩ potentiometers in stock at Digikey was harder than I thought, so I used a 1000uF capacitor and 1MΩ potentiometer instead. You might find one elsewhere, but I didn't feel like hunting.

U1 is a 5VDC regulator. You'll connect your 12VDC supply to the input then use the 5VDC output for the circuit itself.

I've added D1 for reverse voltage protection - if someone connects the 12VDC source incorrectly, this will protect the circuit. If this is not a concern, you can leave it off.

R3 and C5 only trigger the timer on rising signals. This means if the motion sensor is tripped and stays tripped continuously beyond the 10-15 minute timing cycle, the relay will turn off. If this is undesirable, you can simply remove R3 and C5 - R2/Q1 is then connected directly to pin 2 of the 555.

I can recommend a relay, but first I need to know the voltage and current of your CRT. Is it 6.3VDC or 6.3VAC? What is the maximum current draw of the heater?

 

Thanks elec_mech. The schematic looks like it will work nicely. As far as the relay; the heater voltage is 6.3vAC, with max. current of 300mA. (DP10-14 CRT). I really appreciate the help. Tim

 

This should do it. It should fit nicely on the PCB as well.

 

I bet turning the heater on and off might actually shorten the life of the tube- thermal shock kills filaments. Phosphor burn is the more likely wear-out mechanism, blank the beam instead.

 

Sensacell, would I use the same circuit to cut off the beam?

 

A relay output should enable you to interface with just about anything.
You could kill the HV supply (on the primary side of course) or maybe shut off the gun by biasing one of the grids positive.

 

elec mech, seems I am having trouble with this circuit. I have checked my wiring umpteen times and find no faults. I have 4.8v at pin 8 of the 555. The PIR seems to trigger the timer (555), but I only get 2.6v at the relay which does not trigger the relay. I have checked the relay with 5v and it works fine. I get 3.3v at pin 3 of the 555 and 2.6v at the other side of D2. The PIR (red led inside) lights when motion is detected, but goes out after about 1-2 seconds. Adjusting vr1 has no effect. Is it possible to have vr1 wired backwards? (doubtful, but I am puzzled). I have tried another 555, same effect. I did not use R3 and C5, and connected R2/Q1 directly to pin 2. Any ideas? Thanks for your help. Tim

 

Something is wrong with the output voltage, but I can't immediately see what. Are you using a standard 555 or a CMOS version? If you're not sure, let us know what the part number on the 555 IC is.

Adjusting VR1 has no effect on the PIR. The PIR will trigger the 555 and VR1 is used to determine how long you want the 555 output on (relay) once triggered.

Try removing the relay and see what the voltage is before and after D2. Is it higher than 3.3V and 2.6V, respectively? What is the voltage now at pin 8 when a) the circuit is not triggered and b) when it is triggered?

Replace D2 with a diffused red or green LED and a ~270Ω resistor in series going to ground.

What is the voltage at pin 3 now?

Now play with VR1 to "see" the time the relay would go on once the PIR is triggered.

The above steps are to help us determine if the relay coil requires more current than the 555 can provide.

We can either attempt to supply the 555 with more voltage to increase the output voltage or we can use a transistor between the 555 and relay, but let's see what results you get from the above steps first.

 

Depending on current and temperature, with a 4.8V supply, 3.3 to 3.4V out of pin 3 is about what you would expect from a 555.

 

Here is what I have found:
Relay out of circuit, 555 triggered:
before diode: 4.68v.
after diode: 5.64v.
voltage@ pin 8 not triggered: 4.7v.
triggered: 4.97v.
LED in circuit and lit: 3.59v. @ pin 3
555 p/n: NE555P 37aebvm
Yes vr1 does control the length of time the diode stays lit, up to about 13 min. so far. Which brings me to a question; will the PIR automatically reset the timer each time it is triggered? In other words, if it is set at 10 minutes and triggered and movement is detected after 5 minutes, will the timer reset to 10 minutes again in order to keep the timer from shutting off after the initial 10 minutes?

 

Depending on current and temperature, with a 4.8V supply, 3.3 to 3.4V out of pin 3 is about what you would expect from a 555.

Doh! Apparently it's been too long since I played with a 555 - I didn't realize there was a voltage drop on the output.

The PIR will accept up to 6VDC. The simplest solution is to replace U1 with a 7806 to supply 6VDC to the 555. This will raise the output to 4.3VDC which will turn on the relay. While D2 would be ideal to have, you can leave it off and see if it affects operation.

Alternately, you can:

1. Configure a LM317 to get 6VDC if you have one handy.
2. Add a logic level MOSFET between the 555 and the relay coil.
3. Add a BJT transistor between the 555 and relay coil.

will the PIR automatically reset the timer each time it is triggered? In other words, if it is set at 10 minutes and triggered and movement is detected after 5 minutes, will the timer reset to 10 minutes again in order to keep the timer from shutting off after the initial 10 minutes?

No. With R3 and C5 removed, the timer will run for 10 minutes and shut off unless the PIR is triggered at the 10 minute mark. If this is the case, the output of the timer will stay high until the PIR turns off again. When the PIR is triggered again, a new 10 minute cycle will begin.

To do what your describe, you'd have to reset then immediately trigger the 555 again every time the PIR output goes from low to high. I can't immediately think of a quick way to do this, but perhaps with an RC time delay or another 555. If you went this route, I don't think you can keep the relay on if the PIR is triggered continuously, but I may be wrong.

 

I can't immediately think of a quick way to do this,

If you want a re-triggerable monostable function you can arrange for the trigger pulse to discharge the timing capacitor. This may, depending on your triggering circuit, need just an additional diode.

 

alec I appreciate the advice. But I would need to be walked thru this. I am using elec mech's circuit as he has posted here. Thanks.

 

Would it be just as easy to go with a lower coil voltage relay (3 or 4 volt) such as the Z2204-ND (3v.) at digikey?

 

Would it be just as easy to go with a lower coil voltage relay (3 or 4 volt) such as the Z2204-ND (3v.) at digikey?

Well, if you want to make it super easy . . . that would work too.

 

Thanks EM I will order a 3 and 4 v. from DK, plus a 7806 just in case. I have been building this clock for over a year now. Clock works great, just trying to finish up the extra doodads. Do you have any thoughts on Alec_T's idea ? This is the scenario: It is a clock built with a 4" CRT. I want the CRT to power down when no one is around to see it in order to extend the life of the $90.00 CRT. If I shut the entire clock down, it all has to be reset. So I am looking to just shut the CRT down. But I don't want the heaters turning off and on every 10-15 minutes either. Make sense?

 

alec I appreciate the advice. But I would need to be walked thru this.

Here's the mod of elec_mech's circuit which should work providing the output pulses from the PIR are long enough (~300mS) to allow the timing cap C6 to discharge fully. Diode D1 and Q1 do the discharging.
Note that I have added a resistor Rdis to limit the discharge current, and have increased the C3 value to help keep the supply voltage up during peak current pulses.